feat: integrate DB with Open/Close/Put/Delete/Get

- Add db.go with DB struct: Open/Close/Put/Delete/Get/GetDurableSequence/IsWriteStopped
- Add db_test.go with lifecycle, put/get, delete, recovery, and corruption tests
- Extract sentinel errors to errkit/ leaf package to break import cycle (wal -> go_kv)
- Update wal/sequence.go, wal/writer.go to import errkit instead of root package
- Root errors.go re-exports from errkit for backward compatibility

Phase 1 Wave 4d complete (T19).

wal/sequence.go

@@ -3,7 +3,7 @@ package wal
 import (
 	"sync/atomic"
 
-	"github.com/dailz/go-kv"
+	"github.com/dailz/go-kv/errkit"
 )
 
 // SequenceManager manages monotonic sequence number allocation for the WAL.
@@ -29,16 +29,16 @@ func NewSequenceManager(recoveredSequence uint64) *SequenceManager {
 // Returns ErrSequenceExhausted if count is 0 or the allocation would overflow uint64.
 func (sm *SequenceManager) AllocateBatch(count uint32) (baseSequence uint64, err error) {
 	if count == 0 {
-		return 0, go_kv.ErrSequenceExhausted
+		return 0, errkit.ErrSequenceExhausted
 	}
 	for {
 		if sm.exhausted.Load() {
-			return 0, go_kv.ErrSequenceExhausted
+			return 0, errkit.ErrSequenceExhausted
 		}
 		base := sm.nextSequence.Load()
 		last := base + uint64(count) - 1
 		if last < base {
-			return 0, go_kv.ErrSequenceExhausted
+			return 0, errkit.ErrSequenceExhausted
 		}
 		newNext := last + 1
 		if !sm.nextSequence.CompareAndSwap(base, newNext) {