test: add end-to-end tests and benchmark suite

- Add db_e2e_test.go: WAL roundtrip, partial write recovery, multi-segment
  recovery, Get semantics, concurrent read/write, large batch rotation tests
- Add bench_test.go: SinglePut, ConcurrentPut, Get, ConcurrentGet,
  MixedReadWrite, WALRecovery benchmarks with b.ReportAllocs()

Wave 5 complete (T20 + T21). All implementation tasks done.

.omo/plans/phase1-wal.md

@@ -1525,7 +1525,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES
   - Message: `feat: integrate DB with Open/Close/Put/Delete/Get`
 
-- [ ] 20. End-to-end + crash recovery tests
+- [x] 20. End-to-end + crash recovery tests
 
   **What to do**:
   - 创建 `db_test.go`：集成测试
@@ -1591,7 +1591,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES
   - Message: `test: add end-to-end and crash recovery tests`
 
-- [ ] 21. Benchmark suite
+- [x] 21. Benchmark suite
 
   **What to do**:
   - 创建 `bench_test.go`：性能基准