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/writer.go

@@ -6,7 +6,7 @@ import (
 	"sync/atomic"
 	"time"
 
-	go_kv "github.com/dailz/go-kv"
+	"github.com/dailz/go-kv/errkit"
 	"github.com/dailz/go-kv/config"
 	"github.com/dailz/go-kv/memtable"
 )
@@ -131,7 +131,7 @@ func (ww *WalWriter) processRemaining() {
 
 func (ww *WalWriter) processBatch(requests []*CommitRequest) {
 	if ww.writeStopped.Load() {
-		ww.sendError(requests, go_kv.ErrWriteStopped)
+		ww.sendError(requests, errkit.ErrWriteStopped)
 		return
 	}
 
@@ -159,20 +159,20 @@ func (ww *WalWriter) processBatch(requests []*CommitRequest) {
 	}
 
 	if err := ww.segManager.AppendBatch(encoded); err != nil {
-		ww.stopWithError(requests, go_kv.ErrCommitUnknown)
+		ww.stopWithError(requests, errkit.ErrCommitUnknown)
 		return
 	}
 
 	active := ww.memTables.GetActive()
 	if err := writePending(active, entries, baseSequence); err != nil {
 		abortRange(active, baseSequence, lastSequence)
-		ww.stopWithError(requests, go_kv.ErrCommitUnknown)
+		ww.stopWithError(requests, errkit.ErrCommitUnknown)
 		return
 	}
 
 	if err := ww.segManager.Sync(); err != nil {
 		abortRange(active, baseSequence, lastSequence)
-		ww.stopWithError(requests, go_kv.ErrCommitUnknown)
+		ww.stopWithError(requests, errkit.ErrCommitUnknown)
 		return
 	}
 
@@ -263,7 +263,7 @@ func (ww *WalWriter) sendSuccess(requests []*CommitRequest, baseSequence uint64,
 // Put stores key with value.
 func (ww *WalWriter) Put(key, value []byte) error {
 	if ww.writeStopped.Load() {
-		return go_kv.ErrWriteStopped
+		return errkit.ErrWriteStopped
 	}
 
 	req := ww.queue.Submit([]*WalEntry{{
@@ -279,7 +279,7 @@ func (ww *WalWriter) Put(key, value []byte) error {
 // Delete removes key.
 func (ww *WalWriter) Delete(key []byte) error {
 	if ww.writeStopped.Load() {
-		return go_kv.ErrWriteStopped
+		return errkit.ErrWriteStopped
 	}
 
 	req := ww.queue.Submit([]*WalEntry{{