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.go

@@ -0,0 +1,52 @@
+package manifest
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+// Manifest holds database metadata used for recovery.
+// The MANIFEST file stores the recovery checkpoint so that
+// recovery knows which segments are already confirmed durable.
+type Manifest struct {
+	RecoverySegmentID uint64
+}
+
+// Load reads the MANIFEST file from dir.
+// If the file does not exist, it returns a zero-value Manifest with no error (fresh DB).
+func Load(dir string) (*Manifest, error) {
+	path := dir + "/MANIFEST"
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return &Manifest{RecoverySegmentID: 0}, nil
+		}
+		return nil, fmt.Errorf("read manifest: %w", err)
+	}
+
+	line := strings.TrimSpace(string(data))
+	if !strings.HasPrefix(line, "recovery_segment_id:") {
+		return nil, fmt.Errorf("manifest: invalid format: %q", line)
+	}
+	idStr := strings.TrimPrefix(line, "recovery_segment_id:")
+	id, err := strconv.ParseUint(idStr, 10, 64)
+	if err != nil {
+		return nil, fmt.Errorf("manifest: parse recovery_segment_id: %w", err)
+	}
+	return &Manifest{RecoverySegmentID: id}, nil
+}
+
+// Save atomically writes the MANIFEST file to dir with the given recoverySegmentID.
+func Save(dir string, recoverySegmentID uint64) error {
+	content := fmt.Sprintf("recovery_segment_id:%d\n", recoverySegmentID)
+	tmpPath := dir + "/MANIFEST.tmp"
+	if err := os.WriteFile(tmpPath, []byte(content), 0o644); err != nil {
+		return fmt.Errorf("write manifest tmp: %w", err)
+	}
+	if err := os.Rename(tmpPath, dir+"/MANIFEST"); err != nil {
+		return fmt.Errorf("rename manifest: %w", err)
+	}
+	return nil
+}