diff --git a/docs/architecture.md b/docs/architecture.md
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+++ b/docs/architecture.md
@@ -0,0 +1,277 @@
+---
+title: Architecture
+description: Internal design of go-io — the Medium interface, backend implementations, sigil transformation pipeline, and security model.
+---
+
+# Architecture
+
+This document explains how `go-io` is structured internally, how the key types relate to one another, and how data flows through the system.
+
+
+## Design Principles
+
+1. **One interface, many backends.** All storage operations go through `Medium`. Business logic never imports a specific backend.
+2. **Path sandboxing by default.** The local backend validates every path against symlink escapes. Sandboxed mediums cannot reach outside their root.
+3. **Composable transforms.** The `sigil` package lets you chain encoding, compression, and encryption steps into reversible pipelines.
+4. **No CGO.** The SQLite driver (`modernc.org/sqlite`) is pure Go. The entire module compiles without a C toolchain.
+
+
+## Core Types
+
+### Medium (root package)
+
+The `Medium` interface is defined in `io.go`. It is the only type that consuming code needs to know about. The root package also provides:
+
+- **`io.Local`** — a package-level variable initialised in `init()` via `local.New("/")`. This gives unsandboxed access to the host filesystem, mirroring the behaviour of the standard `os` package.
+- **`io.NewSandboxed(root)`** — creates a `local.Medium` restricted to `root`. All path resolution is confined within that directory.
+- **`io.Copy(src, srcPath, dst, dstPath)`** — copies a file between any two mediums by reading from one and writing to the other.
+- **`io.MockMedium`** — a fully functional in-memory implementation for unit tests. It tracks files, directories, and modification times in plain maps.
+
+### FileInfo and DirEntry (root package)
+
+Simple struct implementations of `fs.FileInfo` and `fs.DirEntry` are exported from the root package for use in mocks and tests. Each backend also defines its own unexported equivalents internally.
+
+
+## Backend Implementations
+
+### local.Medium
+
+**File:** `local/client.go`
+
+The local backend wraps the standard `os` package with two layers of path protection:
+
+1. **`path(p string)`** — normalises paths using `filepath.Clean("/" + p)` before joining with the root. This neutralises `..` traversal at the string level. When root is `"/"`, absolute paths pass through unchanged and relative paths resolve against the working directory.
+
+2. **`validatePath(p string)`** — walks each path component, calling `filepath.EvalSymlinks` at every step. If any resolved component lands outside the root, it logs a security event to stderr (including timestamp, root, attempted path, and OS username) and returns `os.ErrPermission`.
+
+Delete operations refuse to remove `"/"` or the user's home directory as an additional safety rail.
+
+```
+caller -> path(p) -> validatePath(p) -> os.ReadFile / os.WriteFile / ...
+```
+
+### s3.Medium
+
+**File:** `s3/s3.go`
+
+The S3 backend translates `Medium` operations into AWS SDK calls. Key design decisions:
+
+- **Key construction:** `key(p)` uses `path.Clean("/" + p)` to sandbox traversal, then prepends the optional prefix. This means `../secret` resolves to `secret`, not an escape.
+- **Directory semantics:** S3 has no real directories. `EnsureDir` is a no-op. `IsDir` and `Exists` for directory-like paths use `ListObjectsV2` with `MaxKeys: 1` to check for objects under the prefix.
+- **Rename:** Implemented as copy-then-delete, since S3 has no atomic rename.
+- **Append:** Downloads existing content, appends in memory, re-uploads on `Close()`. This is the only viable approach given S3's immutable-object model.
+- **Testability:** The `s3API` interface (unexported) abstracts the six SDK methods used. Tests inject a `mockS3` that stores objects in a `map[string][]byte` with a `sync.RWMutex`.
+
+### sqlite.Medium
+
+**File:** `sqlite/sqlite.go`
+
+Stores files and directories as rows in a single SQLite table:
+
+```sql
+CREATE TABLE IF NOT EXISTS files (
+    path    TEXT PRIMARY KEY,
+    content BLOB NOT NULL,
+    mode    INTEGER DEFAULT 420,   -- 0644
+    is_dir  BOOLEAN DEFAULT FALSE,
+    mtime   DATETIME DEFAULT CURRENT_TIMESTAMP
+)
+```
+
+- **WAL mode** is enabled at connection time for better concurrent read performance.
+- **Path cleaning** uses the same `path.Clean("/" + p)` pattern as other backends.
+- **Rename** is transactional: it reads the source row, inserts at the destination, deletes the source, and moves all children (if it is a directory) within a single transaction.
+- **Custom tables** are supported via `WithTable("name")` to allow multiple logical filesystems in one database.
+- **`:memory:`** databases work out of the box for tests.
+
+### node.Node
+
+**File:** `node/node.go`
+
+A pure in-memory filesystem that implements both `Medium` and the standard library's `fs.FS`, `fs.StatFS`, `fs.ReadDirFS`, and `fs.ReadFileFS` interfaces. Directories are implicit -- they exist whenever a stored file path contains a `"/"`.
+
+Key capabilities beyond `Medium`:
+
+- **`ToTar()` / `FromTar()`** — serialise the entire tree to a tar archive and back. This enables snapshotting, transport, and archival.
+- **`Walk()` with `WalkOptions`** — extends `fs.WalkDir` with `MaxDepth`, `Filter`, and `SkipErrors` controls.
+- **`CopyFile(src, dst, perm)`** — copies a file from the in-memory tree to the real filesystem.
+- **`CopyTo(target Medium, src, dst)`** — copies a file or directory tree to any other `Medium`.
+- **`ReadFile(name)`** — returns a defensive copy of file content, preventing callers from mutating internal state.
+
+### datanode.Medium
+
+**File:** `datanode/client.go`
+
+A thread-safe `Medium` backed by Borg's `DataNode` (an in-memory `fs.FS` with tar serialisation). It adds:
+
+- **`sync.RWMutex`** on every operation for concurrent safety.
+- **Explicit directory tracking** via a `map[string]bool`, since `DataNode` only stores files.
+- **`Snapshot()` / `Restore(data)`** — serialise and deserialise the entire filesystem as a tarball.
+- **`DataNode()`** — exposes the underlying Borg DataNode for integration with TIM containers.
+- **File deletion** is handled by rebuilding the DataNode without the target file, since the upstream type does not expose a `Remove` method.
+
+
+## store Package
+
+**Files:** `store/store.go`, `store/medium.go`
+
+The store package provides two complementary APIs:
+
+### Store (key-value)
+
+A group-namespaced key-value store backed by SQLite:
+
+```sql
+CREATE TABLE IF NOT EXISTS kv (
+    grp   TEXT NOT NULL,
+    key   TEXT NOT NULL,
+    value TEXT NOT NULL,
+    PRIMARY KEY (grp, key)
+)
+```
+
+Operations: `Get`, `Set`, `Delete`, `Count`, `DeleteGroup`, `GetAll`, `Render`.
+
+The `Render` method loads all key-value pairs from a group into a `map[string]string` and executes a Go `text/template` against them:
+
+```go
+s.Set("user", "pool", "pool.lthn.io:3333")
+s.Set("user", "wallet", "iz...")
+out, _ := s.Render(`{"pool":"{{ .pool }}"}`, "user")
+// out: {"pool":"pool.lthn.io:3333"}
+```
+
+### store.Medium (Medium adapter)
+
+Wraps a `Store` to satisfy the `Medium` interface. Paths are split as `group/key`:
+
+- `Read("config/theme")` calls `Get("config", "theme")`
+- `List("")` returns all groups as directories
+- `List("config")` returns all keys in the `config` group as files
+- `IsDir("config")` returns true if the group has entries
+
+You can create it directly (`NewMedium(":memory:")`) or adapt an existing store (`store.AsMedium()`).
+
+
+## sigil Package
+
+**Files:** `sigil/sigil.go`, `sigil/sigils.go`, `sigil/crypto_sigil.go`
+
+The sigil package implements composable, reversible data transformations.
+
+### Interface
+
+```go
+type Sigil interface {
+    In(data []byte) ([]byte, error)   // forward transform
+    Out(data []byte) ([]byte, error)  // reverse transform
+}
+```
+
+Contracts:
+- Reversible sigils: `Out(In(x)) == x`
+- Irreversible sigils (hashes): `Out` returns input unchanged
+- Symmetric sigils (reverse): `In(x) == Out(x)`
+- `nil` input returns `nil` without error
+- Empty input returns empty without error
+
+### Available Sigils
+
+Created via `NewSigil(name)`:
+
+| Name | Type | Reversible |
+|------|------|------------|
+| `reverse` | Byte reversal | Yes (symmetric) |
+| `hex` | Hexadecimal encoding | Yes |
+| `base64` | Base64 encoding | Yes |
+| `gzip` | Gzip compression | Yes |
+| `json` | JSON compaction | No (`Out` is passthrough) |
+| `json-indent` | JSON pretty-printing | No (`Out` is passthrough) |
+| `md4`, `md5`, `sha1` | Legacy hashes | No |
+| `sha224` .. `sha512` | SHA-2 family | No |
+| `sha3-224` .. `sha3-512` | SHA-3 family | No |
+| `sha512-224`, `sha512-256` | Truncated SHA-512 | No |
+| `ripemd160` | RIPEMD-160 | No |
+| `blake2s-256` | BLAKE2s | No |
+| `blake2b-256` .. `blake2b-512` | BLAKE2b | No |
+
+### Pipeline Functions
+
+```go
+// Apply sigils left-to-right.
+encoded, _ := sigil.Transmute(data, []sigil.Sigil{gzipSigil, hexSigil})
+
+// Reverse sigils right-to-left.
+original, _ := sigil.Untransmute(encoded, []sigil.Sigil{gzipSigil, hexSigil})
+```
+
+### Authenticated Encryption: ChaChaPolySigil
+
+`ChaChaPolySigil` provides XChaCha20-Poly1305 authenticated encryption with a pre-obfuscation layer. It implements the `Sigil` interface, so it composes naturally into pipelines.
+
+**Encryption flow:**
+
+```
+plaintext -> obfuscate(nonce) -> XChaCha20-Poly1305 encrypt -> [nonce || ciphertext || tag]
+```
+
+**Decryption flow:**
+
+```
+[nonce || ciphertext || tag] -> decrypt -> deobfuscate(nonce) -> plaintext
+```
+
+The pre-obfuscation layer ensures that raw plaintext patterns are never sent directly to CPU encryption routines, providing defence-in-depth against side-channel attacks. Two obfuscators are provided:
+
+- **`XORObfuscator`** (default) — XORs data with a SHA-256 counter-mode key stream derived from the nonce.
+- **`ShuffleMaskObfuscator`** — applies XOR masking followed by a deterministic Fisher-Yates byte shuffle, making both value and position analysis more difficult.
+
+```go
+key := make([]byte, 32)
+rand.Read(key)
+
+s, _ := sigil.NewChaChaPolySigil(key)
+ciphertext, _ := s.In([]byte("secret"))
+plaintext, _ := s.Out(ciphertext)
+
+// With stronger obfuscation:
+s2, _ := sigil.NewChaChaPolySigilWithObfuscator(key, &sigil.ShuffleMaskObfuscator{})
+```
+
+Each call to `In` generates a fresh random nonce, so encrypting the same plaintext twice produces different ciphertexts.
+
+
+## workspace Package
+
+**File:** `workspace/service.go`
+
+A higher-level service that integrates with the Core DI container (`forge.lthn.ai/core/go`). It manages encrypted workspaces stored under `~/.core/workspaces/`.
+
+Each workspace:
+- Is identified by a SHA-256 hash of the user-provided identifier
+- Contains subdirectories: `config/`, `log/`, `data/`, `files/`, `keys/`
+- Has a PGP keypair generated via the Core crypt service
+- Supports file get/set operations on the `files/` subdirectory
+- Handles IPC events (`workspace.create`, `workspace.switch`) for integration with the Core message bus
+
+The workspace service implements `core.Workspace` and uses `io.Local` as its storage medium.
+
+
+## Data Flow Summary
+
+```
+Application code
+       |
+       v
+   io.Medium (interface)
+       |
+       +-- local.Medium  --> os package (with sandbox validation)
+       +-- s3.Medium     --> AWS SDK S3 client
+       +-- sqlite.Medium --> modernc.org/sqlite
+       +-- node.Node     --> in-memory map + tar serialisation
+       +-- datanode.Medium --> Borg DataNode + sync.RWMutex
+       +-- store.Medium  --> store.Store (SQLite KV) --> Medium adapter
+       +-- MockMedium    --> map[string]string (for tests)
+```
+
+Every backend normalises paths using the same `path.Clean("/" + p)` pattern, ensuring consistent behaviour regardless of which backend is in use.
diff --git a/docs/development.md b/docs/development.md
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--- /dev/null
+++ b/docs/development.md
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+---
+title: Development
+description: How to build, test, and contribute to go-io.
+---
+
+# Development
+
+This guide covers everything needed to work on `go-io` locally.
+
+
+## Prerequisites
+
+- **Go 1.26.0** or later
+- **No C compiler required** -- all dependencies (including SQLite) are pure Go
+- The module is part of the Go workspace at `~/Code/go.work`. If you are working outside that workspace, ensure `GOPRIVATE=forge.lthn.ai/*` is set so the Go toolchain can fetch private dependencies.
+
+
+## Building
+
+`go-io` is a library with no binary output. To verify it compiles:
+
+```bash
+cd /path/to/go-io
+go build ./...
+```
+
+If using the Core CLI:
+
+```bash
+core go fmt       # format
+core go vet       # static analysis
+core go lint      # linter
+core go test      # run all tests
+core go qa        # fmt + vet + lint + test
+core go qa full   # + race detector, vulnerability scan, security audit
+```
+
+
+## Running Tests
+
+All packages have thorough test suites. Tests use `testify/assert` and `testify/require` for assertions.
+
+```bash
+# All tests
+go test ./...
+
+# A single package
+go test ./sigil/
+
+# A single test by name
+go test ./local/ -run TestValidatePath_Security
+
+# With race detector
+go test -race ./...
+
+# With coverage
+go test -coverprofile=coverage.out ./...
+go tool cover -html=coverage.out
+```
+
+Or via the Core CLI:
+
+```bash
+core go test
+core go test --run TestChaChaPolySigil_Good_RoundTrip
+core go cov --open
+```
+
+
+## Test Naming Convention
+
+Tests follow the `_Good`, `_Bad`, `_Ugly` suffix pattern:
+
+| Suffix | Meaning |
+|--------|---------|
+| `_Good` | Happy path -- the operation succeeds as expected |
+| `_Bad` | Expected error conditions -- missing files, invalid input, permission denied |
+| `_Ugly` | Edge cases and boundary conditions -- nil input, empty paths, panics |
+
+Example:
+
+```go
+func TestDelete_Good(t *testing.T)           { /* deletes a file successfully */ }
+func TestDelete_Bad_NotFound(t *testing.T)   { /* returns error for missing file */ }
+func TestDelete_Bad_DirNotEmpty(t *testing.T) { /* returns error for non-empty dir */ }
+```
+
+
+## Writing Tests Against Medium
+
+Use `MockMedium` from the root package for unit tests that need a storage backend but should not touch disk:
+
+```go
+func TestMyFeature(t *testing.T) {
+    m := io.NewMockMedium()
+    m.Files["config.yaml"] = "key: value"
+    m.Dirs["data"] = true
+
+    // Your code under test receives m as an io.Medium
+    result, err := myFunction(m)
+    assert.NoError(t, err)
+    assert.Equal(t, "expected", m.Files["output.txt"])
+}
+```
+
+For tests that need a real but ephemeral filesystem, use `local.New` with `t.TempDir()`:
+
+```go
+func TestWithRealFS(t *testing.T) {
+    m, err := local.New(t.TempDir())
+    require.NoError(t, err)
+
+    _ = m.Write("file.txt", "hello")
+    content, _ := m.Read("file.txt")
+    assert.Equal(t, "hello", content)
+}
+```
+
+For SQLite-backed tests, use `:memory:`:
+
+```go
+func TestWithSQLite(t *testing.T) {
+    m, err := sqlite.New(":memory:")
+    require.NoError(t, err)
+    defer m.Close()
+
+    _ = m.Write("file.txt", "hello")
+}
+```
+
+
+## Adding a New Backend
+
+To add a new `Medium` implementation:
+
+1. Create a new package directory (e.g., `sftp/`).
+2. Define a struct that implements all 18 methods of `io.Medium`.
+3. Add a compile-time check at the top of your file:
+
+```go
+var _ coreio.Medium = (*Medium)(nil)
+```
+
+4. Normalise paths using `path.Clean("/" + p)` to prevent traversal escapes. This is the convention followed by every existing backend.
+5. Handle `nil` and empty input consistently: check how `MockMedium` and `local.Medium` behave and match that behaviour.
+6. Write tests using the `_Good` / `_Bad` / `_Ugly` naming convention.
+7. Add your package to the table in `docs/index.md`.
+
+
+## Adding a New Sigil
+
+To add a new data transformation:
+
+1. Create a struct in `sigil/` that implements the `Sigil` interface (`In` and `Out`).
+2. Handle `nil` input by returning `nil, nil`.
+3. Handle empty input by returning `[]byte{}, nil`.
+4. Register it in the `NewSigil` factory function in `sigils.go`.
+5. Add tests covering `_Good` (round-trip), `_Bad` (invalid input), and `_Ugly` (nil/empty edge cases).
+
+
+## Code Style
+
+- **UK English** in comments and documentation: colour, organisation, centre, serialise, defence.
+- **`declare(strict_types=1)`** equivalent: all functions have explicit parameter and return types.
+- Errors use the `go-log` helper: `coreerr.E("package.Method", "what failed", underlyingErr)`.
+- No blank imports except for database drivers (`_ "modernc.org/sqlite"`).
+- Formatting: standard `gofmt` / `goimports`.
+
+
+## Project Structure
+
+```
+go-io/
+├── io.go               # Medium interface, helpers, MockMedium
+├── client_test.go      # Tests for MockMedium and helpers
+├── bench_test.go       # Benchmarks
+├── go.mod
+├── local/
+│   ├── client.go       # Local filesystem backend
+│   └── client_test.go
+├── s3/
+│   ├── s3.go           # S3 backend
+│   └── s3_test.go
+├── sqlite/
+│   ├── sqlite.go       # SQLite virtual filesystem
+│   └── sqlite_test.go
+├── node/
+│   ├── node.go         # In-memory fs.FS + Medium
+│   └── node_test.go
+├── datanode/
+│   ├── client.go       # Borg DataNode Medium wrapper
+│   └── client_test.go
+├── store/
+│   ├── store.go        # KV store
+│   ├── medium.go       # Medium adapter for KV store
+│   ├── store_test.go
+│   └── medium_test.go
+├── sigil/
+│   ├── sigil.go        # Sigil interface, Transmute/Untransmute
+│   ├── sigils.go       # Built-in sigils (hex, base64, gzip, hash, etc.)
+│   ├── crypto_sigil.go # ChaChaPolySigil + obfuscators
+│   ├── sigil_test.go
+│   └── crypto_sigil_test.go
+├── workspace/
+│   ├── service.go      # Encrypted workspace service
+│   └── service_test.go
+├── docs/               # This documentation
+└── .core/
+    ├── build.yaml      # Build configuration
+    └── release.yaml    # Release configuration
+```
+
+
+## Licence
+
+EUPL-1.2
diff --git a/docs/index.md b/docs/index.md
new file mode 100644
index 0000000..bd33262
--- /dev/null
+++ b/docs/index.md
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+---
+title: go-io
+description: Unified storage abstraction for Go with pluggable backends — local filesystem, S3, SQLite, in-memory, and key-value.
+---
+
+# go-io
+
+`forge.lthn.ai/core/go-io` is a storage abstraction library that provides a single `Medium` interface for reading and writing files across different backends. Write your code against `Medium` once, then swap between local disk, S3, SQLite, or in-memory storage without changing a line of business logic.
+
+The library also includes `sigil`, a composable data-transformation pipeline for encoding, compression, hashing, and authenticated encryption.
+
+
+## Quick Start
+
+```go
+import (
+    io "forge.lthn.ai/core/go-io"
+    "forge.lthn.ai/core/go-io/s3"
+    "forge.lthn.ai/core/go-io/node"
+)
+
+// Use the pre-initialised local filesystem (unsandboxed, rooted at "/").
+content, _ := io.Local.Read("/etc/hostname")
+
+// Create a sandboxed medium restricted to a single directory.
+sandbox, _ := io.NewSandboxed("/var/data/myapp")
+_ = sandbox.Write("config.yaml", "key: value")
+
+// In-memory filesystem with tar serialisation.
+mem := node.New()
+mem.AddData("hello.txt", []byte("world"))
+tarball, _ := mem.ToTar()
+
+// S3 backend (requires an *s3.Client from the AWS SDK).
+bucket, _ := s3.New("my-bucket", s3.WithClient(awsClient), s3.WithPrefix("uploads/"))
+_ = bucket.Write("photo.jpg", rawData)
+```
+
+
+## Package Layout
+
+| Package | Import Path | Purpose |
+|---------|-------------|---------|
+| `io` (root) | `forge.lthn.ai/core/go-io` | `Medium` interface, helper functions, `MockMedium` for tests |
+| `local` | `forge.lthn.ai/core/go-io/local` | Local filesystem backend with path sandboxing and symlink-escape protection |
+| `s3` | `forge.lthn.ai/core/go-io/s3` | Amazon S3 / S3-compatible backend (Garage, MinIO, etc.) |
+| `sqlite` | `forge.lthn.ai/core/go-io/sqlite` | SQLite-backed virtual filesystem (pure Go driver, no CGO) |
+| `node` | `forge.lthn.ai/core/go-io/node` | In-memory filesystem implementing both `Medium` and `fs.FS`, with tar round-tripping |
+| `datanode` | `forge.lthn.ai/core/go-io/datanode` | Thread-safe in-memory `Medium` backed by Borg's DataNode, with snapshot/restore |
+| `store` | `forge.lthn.ai/core/go-io/store` | Group-namespaced key-value store (SQLite), with a `Medium` adapter and Go template rendering |
+| `sigil` | `forge.lthn.ai/core/go-io/sigil` | Composable data transformations: encoding, compression, hashing, XChaCha20-Poly1305 encryption |
+| `workspace` | `forge.lthn.ai/core/go-io/workspace` | Encrypted workspace service integrated with the Core DI container |
+
+
+## The Medium Interface
+
+Every storage backend implements the same 18-method interface:
+
+```go
+type Medium interface {
+    // Content operations
+    Read(path string) (string, error)
+    Write(path, content string) error
+    FileGet(path string) (string, error)   // alias for Read
+    FileSet(path, content string) error    // alias for Write
+
+    // Streaming (for large files)
+    ReadStream(path string) (io.ReadCloser, error)
+    WriteStream(path string) (io.WriteCloser, error)
+    Open(path string) (fs.File, error)
+    Create(path string) (io.WriteCloser, error)
+    Append(path string) (io.WriteCloser, error)
+
+    // Directory operations
+    EnsureDir(path string) error
+    List(path string) ([]fs.DirEntry, error)
+
+    // Metadata
+    Stat(path string) (fs.FileInfo, error)
+    Exists(path string) bool
+    IsFile(path string) bool
+    IsDir(path string) bool
+
+    // Mutation
+    Delete(path string) error
+    DeleteAll(path string) error
+    Rename(oldPath, newPath string) error
+}
+```
+
+All backends implement this interface fully. Backends where a method has no natural equivalent (e.g., `EnsureDir` on S3) provide a safe no-op.
+
+
+## Cross-Medium Operations
+
+The root package provides helper functions that accept any `Medium`:
+
+```go
+// Copy a file between any two backends.
+err := io.Copy(localMedium, "source.txt", s3Medium, "dest.txt")
+
+// Read/Write wrappers that take an explicit medium.
+content, err := io.Read(medium, "path")
+err := io.Write(medium, "path", "content")
+```
+
+
+## Dependencies
+
+| Dependency | Role |
+|------------|------|
+| `forge.lthn.ai/core/go-log` | Structured error helper (`E()`) |
+| `forge.lthn.ai/Snider/Borg` | DataNode in-memory FS (used by `datanode` package) |
+| `github.com/aws/aws-sdk-go-v2` | S3 client (used by `s3` package) |
+| `golang.org/x/crypto` | BLAKE2, SHA-3, RIPEMD-160, XChaCha20-Poly1305 (used by `sigil`) |
+| `modernc.org/sqlite` | Pure Go SQLite driver (used by `sqlite` and `store`) |
+| `github.com/stretchr/testify` | Test assertions |
+
+Go version: **1.26.0**
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+Licence: **EUPL-1.2**