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) {

wal/sequence_test.go

@@ -7,7 +7,7 @@ import (
 	"sync/atomic"
 	"testing"
 
-	"github.com/dailz/go-kv"
+	"github.com/dailz/go-kv/errkit"
 )
 
 func TestSequenceAllocation(t *testing.T) {
@@ -49,7 +49,7 @@ func TestSequenceOverflow(t *testing.T) {
 	// Remaining: MaxUint64-2, MaxUint64-1, MaxUint64 = 3 slots.
 	// Asking for 5 should overflow.
 	_, err := sm.AllocateBatch(5)
-	if !errors.Is(err, go_kv.ErrSequenceExhausted) {
+	if !errors.Is(err, errkit.ErrSequenceExhausted) {
 		t.Fatalf("expected ErrSequenceExhausted, got %v", err)
 	}
 
@@ -64,7 +64,7 @@ func TestSequenceOverflow(t *testing.T) {
 
 	// Now any further allocation should fail.
 	_, err = sm.AllocateBatch(1)
-	if !errors.Is(err, go_kv.ErrSequenceExhausted) {
+	if !errors.Is(err, errkit.ErrSequenceExhausted) {
 		t.Fatalf("expected ErrSequenceExhausted after exhaustion, got %v", err)
 	}
 }
@@ -108,7 +108,7 @@ func TestZeroCountRejected(t *testing.T) {
 	sm := NewSequenceManager(0)
 
 	_, err := sm.AllocateBatch(0)
-	if !errors.Is(err, go_kv.ErrSequenceExhausted) {
+	if !errors.Is(err, errkit.ErrSequenceExhausted) {
 		t.Fatalf("expected ErrSequenceExhausted for count=0, got %v", err)
 	}
 }

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{{