feat(wal): implement WAL writer with group commit and recovery batch replay

- wal/commit_queue.go: bounded buffered channel for write requests
- wal/writer.go: single-goroutine main loop implementing 11-step write flow
  with group commit, sequence allocation, MemTable publish/abort, write-stopped
- wal/recovery.go: BatchReplayer interface, ReplayBatch, ReplaySegmentFile,
  RecoverFromSegments with fragment reassembly and tail corruption handling
- Comprehensive tests for all modules, all pass with -race

wal/writer_test.go

@@ -0,0 +1,120 @@
+package wal
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+
+	"github.com/dailz/go-kv/config"
+)
+
+func newTestWalWriter(t *testing.T) *WalWriter {
+	t.Helper()
+
+	cfg := config.Defaults()
+	ww, err := NewWalWriter(&cfg, t.TempDir(), 0, 0)
+	if err != nil {
+		t.Fatalf("NewWalWriter: %v", err)
+	}
+	t.Cleanup(func() {
+		if err := ww.Close(); err != nil {
+			t.Fatalf("Close: %v", err)
+		}
+	})
+
+	return ww
+}
+
+func TestWalWriterSinglePut(t *testing.T) {
+	ww := newTestWalWriter(t)
+
+	if err := ww.Put([]byte("k1"), []byte("v1")); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	got := ww.Get([]byte("k1"))
+	if !got.Found {
+		t.Fatal("Get(k1) not found")
+	}
+	if string(got.Value) != "v1" {
+		t.Fatalf("Get(k1) value = %q, want %q", got.Value, "v1")
+	}
+}
+
+func TestWalWriterMultiplePuts(t *testing.T) {
+	ww := newTestWalWriter(t)
+
+	for i := range 10 {
+		key := fmt.Appendf(nil, "k%d", i)
+		value := fmt.Appendf(nil, "v%d", i)
+		if err := ww.Put(key, value); err != nil {
+			t.Fatalf("Put(%q): %v", key, err)
+		}
+	}
+
+	for i := range 10 {
+		key := fmt.Appendf(nil, "k%d", i)
+		want := fmt.Sprintf("v%d", i)
+		got := ww.Get(key)
+		if !got.Found {
+			t.Fatalf("Get(%q) not found", key)
+		}
+		if string(got.Value) != want {
+			t.Fatalf("Get(%q) value = %q, want %q", key, got.Value, want)
+		}
+	}
+}
+
+func TestWalWriterGroupCommit(t *testing.T) {
+	ww := newTestWalWriter(t)
+
+	const writers = 5
+	var wg sync.WaitGroup
+	errCh := make(chan error, writers)
+
+	for i := range writers {
+		wg.Go(func() {
+			key := fmt.Appendf(nil, "group-k%d", i)
+			value := fmt.Appendf(nil, "group-v%d", i)
+			if err := ww.Put(key, value); err != nil {
+				errCh <- fmt.Errorf("put %d: %w", i, err)
+			}
+		})
+	}
+
+	wg.Wait()
+	close(errCh)
+	for err := range errCh {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	for i := range writers {
+		key := fmt.Appendf(nil, "group-k%d", i)
+		want := fmt.Sprintf("group-v%d", i)
+		got := ww.Get(key)
+		if !got.Found {
+			t.Fatalf("Get(%q) not found", key)
+		}
+		if string(got.Value) != want {
+			t.Fatalf("Get(%q) value = %q, want %q", key, got.Value, want)
+		}
+	}
+}
+
+func TestWalWriterDelete(t *testing.T) {
+	ww := newTestWalWriter(t)
+
+	if err := ww.Put([]byte("k"), []byte("v")); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+	if err := ww.Delete([]byte("k")); err != nil {
+		t.Fatalf("Delete: %v", err)
+	}
+
+	got := ww.Get([]byte("k"))
+	if got.Found {
+		t.Fatalf("Get(k) found deleted key with value %q", got.Value)
+	}
+}