feat: initialize project structure and error types

- go.mod with github.com/dailz/go-kv, Go 1.26.3, testify
- config/config.go with WalConfig, Validate() with checked arithmetic
- errors.go with sentinel errors (ErrCommitUnknown, ErrWriteStopped, etc.)
- wal/constants.go with all WAL format constants and enums
- wal/header.go with WAL File Header encode/decode (CRC32 IEEE)
- wal/record.go with Physical Record codec, block boundary, SplitIntoRecords
- wal/entry.go with WAL Entry codec (varint keys/values, OpType, ValueKind)
- wal/sequence.go with SequenceManager (atomic, CAS, overflow-safe)
- manifest/manifest.go with MANIFEST stub (Load/Save atomic)
- manifest/current.go with CURRENT file (WriteCurrent/ReadCurrent)
- Comprehensive tests for all modules
- .golangci.yml configuration

manifest/manifest_test.go

@@ -0,0 +1,45 @@
+package manifest
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestManifestRoundtrip(t *testing.T) {
+	dir := t.TempDir()
+
+	err := Save(dir, 42)
+	require.NoError(t, err)
+
+	m, err := Load(dir)
+	require.NoError(t, err)
+	assert.Equal(t, uint64(42), m.RecoverySegmentID)
+}
+
+func TestManifestMissing(t *testing.T) {
+	dir := t.TempDir()
+
+	m, err := Load(dir)
+	require.NoError(t, err)
+	assert.Equal(t, uint64(0), m.RecoverySegmentID)
+}
+
+func TestCurrentRoundtrip(t *testing.T) {
+	dir := t.TempDir()
+
+	err := WriteCurrent(dir, 5)
+	require.NoError(t, err)
+
+	segmentID, ok := ReadCurrent(dir)
+	assert.True(t, ok)
+	assert.Equal(t, uint64(5), segmentID)
+}
+
+func TestCurrentMissing(t *testing.T) {
+	dir := t.TempDir()
+
+	_, ok := ReadCurrent(dir)
+	assert.False(t, ok)
+}