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_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)
 	}
 }