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).

errors.go

@@ -1,22 +1,11 @@
 package go_kv
 
-import "errors"
+import "github.com/dailz/go-kv/errkit"
 
-// ErrCommitUnknown indicates the commit result is indeterminate: the WAL write
-// was attempted but the caller cannot assume the write definitely failed or
-// succeeded. The caller must query the commit state before retrying.
-var ErrCommitUnknown = errors.New("go-kv: commit result unknown")
-
-// ErrWriteStopped indicates the engine has entered a write-stopped state and
-// rejects all subsequent writes.
-var ErrWriteStopped = errors.New("go-kv: write stopped")
-
-// ErrSequenceExhausted indicates the sequence number space is exhausted. The
-// engine enters a terminal state requiring database migration or rebuild.
-var ErrSequenceExhausted = errors.New("go-kv: sequence exhausted")
-
-// ErrWALCorrupted indicates WAL data corruption was detected.
-var ErrWALCorrupted = errors.New("go-kv: WAL corrupted")
-
-// ErrInvalidConfig indicates an invalid configuration was provided.
-var ErrInvalidConfig = errors.New("go-kv: invalid config")
+var (
+	ErrCommitUnknown     = errkit.ErrCommitUnknown
+	ErrWriteStopped      = errkit.ErrWriteStopped
+	ErrSequenceExhausted = errkit.ErrSequenceExhausted
+	ErrWALCorrupted      = errkit.ErrWALCorrupted
+	ErrInvalidConfig     = errkit.ErrInvalidConfig
+)