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).

.omo/plans/phase1-wal.md

@@ -1412,7 +1412,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES
   - Message: `feat: implement MANIFEST stub and CURRENT file`
 
-- [ ] 19. DB integration: Open/Close/Put/Delete/Get
+- [x] 19. DB integration: Open/Close/Put/Delete/Get
 
   **What to do**:
   - 创建 `db.go`：`DB` 结构体和核心 API