feat: add zstd compress/decompress helpers for cold storage

Add compressFileZstd, decompressZstd, and walkZstFiles helpers
using klauspost/compress. Promote zstd from indirect to direct dep.

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

pkg/lem/zstd.go

@@ -0,0 +1,77 @@
+// zstd.go — zstd compression/decompression for cold storage JSONL files.
+package lem
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/klauspost/compress/zstd"
+)
+
+// decompressZstd reads a .zst file and writes the decompressed content to dst.
+func decompressZstd(src, dst string) error {
+	f, err := os.Open(src)
+	if err != nil {
+		return fmt.Errorf("open %s: %w", src, err)
+	}
+	defer f.Close()
+
+	dec, err := zstd.NewReader(f)
+	if err != nil {
+		return fmt.Errorf("zstd reader: %w", err)
+	}
+	defer dec.Close()
+
+	out, err := os.Create(dst)
+	if err != nil {
+		return fmt.Errorf("create %s: %w", dst, err)
+	}
+	defer out.Close()
+
+	if _, err := io.Copy(out, dec); err != nil {
+		return fmt.Errorf("decompress %s: %w", src, err)
+	}
+	return nil
+}
+
+// compressFileZstd compresses src to dst with zstd level 3.
+func compressFileZstd(src, dst string) error {
+	f, err := os.Open(src)
+	if err != nil {
+		return fmt.Errorf("open %s: %w", src, err)
+	}
+	defer f.Close()
+
+	out, err := os.Create(dst)
+	if err != nil {
+		return fmt.Errorf("create %s: %w", dst, err)
+	}
+	defer out.Close()
+
+	enc, err := zstd.NewWriter(out, zstd.WithEncoderLevel(zstd.SpeedDefault))
+	if err != nil {
+		return fmt.Errorf("zstd writer: %w", err)
+	}
+
+	if _, err := io.Copy(enc, f); err != nil {
+		enc.Close()
+		return fmt.Errorf("compress %s: %w", src, err)
+	}
+	return enc.Close()
+}
+
+// walkZstFiles walks a directory tree and calls fn for each .jsonl.zst file.
+func walkZstFiles(root string, fn func(zstPath string) error) error {
+	return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
+		if err != nil || info.IsDir() {
+			return err
+		}
+		if strings.HasSuffix(path, ".jsonl.zst") {
+			return fn(path)
+		}
+		return nil
+	})
+}

pkg/lem/zstd_test.go

@@ -0,0 +1,58 @@
+package lem
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestDecompressZstd(t *testing.T) {
+	// Create a temp .jsonl file and compress it
+	dir := t.TempDir()
+	src := filepath.Join(dir, "test.jsonl")
+	os.WriteFile(src, []byte(`{"messages":[{"role":"user","content":"hello"}]}`+"\n"), 0644)
+
+	// Compress with Go zstd
+	zstPath := src + ".zst"
+	if err := compressFileZstd(src, zstPath); err != nil {
+		t.Fatal(err)
+	}
+	os.Remove(src) // remove original
+
+	// Decompress
+	outPath := filepath.Join(dir, "out.jsonl")
+	if err := decompressZstd(zstPath, outPath); err != nil {
+		t.Fatal(err)
+	}
+
+	data, err := os.ReadFile(outPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != `{"messages":[{"role":"user","content":"hello"}]}`+"\n" {
+		t.Fatalf("unexpected content: %q", string(data))
+	}
+}
+
+func TestWalkZstFiles(t *testing.T) {
+	dir := t.TempDir()
+	// Create a nested .jsonl.zst
+	sub := filepath.Join(dir, "sub")
+	os.MkdirAll(sub, 0755)
+	src := filepath.Join(sub, "data.jsonl")
+	os.WriteFile(src, []byte("line1\nline2\n"), 0644)
+	compressFileZstd(src, src+".zst")
+	os.Remove(src)
+
+	// Also create a .json file (should be ignored)
+	os.WriteFile(filepath.Join(sub, "probes.json"), []byte("[]"), 0644)
+
+	var found []string
+	walkZstFiles(dir, func(zstPath string) error {
+		found = append(found, zstPath)
+		return nil
+	})
+	if len(found) != 1 {
+		t.Fatalf("expected 1 .zst file, got %d: %v", len(found), found)
+	}
+}