fix(dx): update CLAUDE.md and improve test coverage

- Fix WASM size gate: 3 MB → 3.5 MB to match size_test.go
- Fix dependencies: remove stale replace directive, add go-io and go-log
- Add error handling and I/O conventions to coding standards
- Add tests: reader error (cmd/codegen), duplicate tag dedup, bundle
  error propagation (codegen) — coverage: codegen 89→96%, cmd/codegen
  71→78%

Co-Authored-By: Virgil <virgil@lethean.io>

CLAUDE.md

@@ -27,7 +27,7 @@ See `docs/architecture.md` for full detail. Summary:
 - **Responsive**: Multi-variant breakpoint wrapper (`data-variant` attributes), renders all variants in insertion order
 - **Pipeline**: Render → StripTags → go-i18n/reversal Tokenise → GrammarImprint (server-side only)
 - **Codegen**: Web Component classes with closed Shadow DOM, generated at build time by `cmd/codegen/`
-- **WASM**: `cmd/wasm/` exports `renderToString()` only — size gate: < 3 MB raw, < 1 MB gzip
+- **WASM**: `cmd/wasm/` exports `renderToString()` only — size gate: < 3.5 MB raw, < 1 MB gzip
 
 ## Server/Client Split
 
@@ -40,9 +40,10 @@ Files guarded with `//go:build !js` are excluded from WASM:
 
 ## Dependencies
 
-- `forge.lthn.ai/core/go-i18n` (replace directive → `../go-i18n`)
+- `forge.lthn.ai/core/go-i18n` — internationalisation, text reversal pipeline
+- `forge.lthn.ai/core/go-io` — file I/O abstraction (`coreio.Local`)
+- `forge.lthn.ai/core/go-log` — structured logging and error wrapping (`log.E()`)
 - `forge.lthn.ai/core/go-inference` (indirect, via go-i18n)
-- Both `go-i18n` and `go-inference` must be cloned alongside this repo for builds
 - Go 1.26+ required (uses `range` over integers, `iter.Seq`, `maps.Keys`, `slices.Collect`)
 
 ## Coding Standards
@@ -53,6 +54,8 @@ Files guarded with `//go:build !js` are excluded from WASM:
 - Licence: EUPL-1.2 — add `// SPDX-Licence-Identifier: EUPL-1.2` to new files
 - Safe-by-default: HTML escaping via `html.EscapeString()` on Text nodes and attribute values, void element handling, entitlement deny-by-default
 - Deterministic output: sorted attributes on El nodes, reproducible block ID paths
+- Error handling: use `log.E(scope, message, err)` from `go-log`, never `fmt.Errorf`
+- File I/O: use `coreio.Local` from `go-io`, never `os.ReadFile`/`os.WriteFile`
 - Commits: conventional commits + `Co-Authored-By: Virgil <virgil@lethean.io>`
 
 ## Test Conventions

cmd/codegen/main_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"strings"
 	"testing"
 
@@ -9,6 +10,12 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+type errReader struct{}
+
+func (errReader) Read([]byte) (int, error) {
+	return 0, errors.New("read failed")
+}
+
 func TestRun_Good(t *testing.T) {
 	input := strings.NewReader(`{"H":"nav-bar","C":"main-content"}`)
 	var output bytes.Buffer
@@ -49,3 +56,11 @@ func TestRun_Good_Empty(t *testing.T) {
 	require.NoError(t, err)
 	assert.Empty(t, output.String())
 }
+
+func TestRun_Bad_ReadError(t *testing.T) {
+	var output bytes.Buffer
+
+	err := run(errReader{}, &output)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "reading stdin")
+}

codegen/codegen_test.go

@@ -52,3 +52,23 @@ func TestGenerateBundle_Good(t *testing.T) {
 	assert.Contains(t, js, "MainContent")
 	assert.Equal(t, 2, strings.Count(js, "extends HTMLElement"))
 }
+
+func TestGenerateBundle_Good_DeduplicatesTags(t *testing.T) {
+	slots := map[string]string{
+		"H": "nav-bar",
+		"F": "nav-bar",
+	}
+	js, err := GenerateBundle(slots)
+	require.NoError(t, err)
+	assert.Equal(t, 1, strings.Count(js, "extends HTMLElement"),
+		"duplicate tag should produce only one class definition")
+}
+
+func TestGenerateBundle_Bad_InvalidTag(t *testing.T) {
+	slots := map[string]string{
+		"H": "notag",
+	}
+	_, err := GenerateBundle(slots)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "hyphen")
+}