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/boulder.json

@@ -8,7 +8,7 @@
       "plan_name": "phase1-wal",
       "status": "active",
       "started_at": "2026-06-12T05:09:32.588Z",
-      "updated_at": "2026-06-12T06:09:22.858Z",
+      "updated_at": "2026-06-12T06:20:06.478Z",
       "session_ids": [
         "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
       ],
@@ -133,6 +133,19 @@
           "status": "completed",
           "ended_at": "2026-06-12T06:09:22.858Z",
           "elapsed_ms": 128738
+        },
+        "todo:20": {
+          "task_key": "todo:20",
+          "task_label": "20",
+          "task_title": "End-to-end + crash recovery tests",
+          "session_id": "opencode:ses_1458aa770ffePqE4ip81RsOmNR",
+          "agent": "Sisyphus-Junior",
+          "category": "deep",
+          "updated_at": "2026-06-12T06:20:06.478Z",
+          "started_at": "2026-06-12T06:18:40.751Z",
+          "status": "completed",
+          "ended_at": "2026-06-12T06:20:06.478Z",
+          "elapsed_ms": 85727
         }
       }
     }
@@ -140,7 +153,7 @@
   "active_plan": "/home/dailz/workspace/src/go-kv/.omo/plans/phase1-wal.md",
   "started_at": "2026-06-12T05:09:32.588Z",
   "status": "active",
-  "updated_at": "2026-06-12T06:09:22.858Z",
+  "updated_at": "2026-06-12T06:20:06.478Z",
   "session_ids": [
     "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
   ],
@@ -265,6 +278,19 @@
       "status": "completed",
       "ended_at": "2026-06-12T06:09:22.858Z",
       "elapsed_ms": 128738
+    },
+    "todo:20": {
+      "task_key": "todo:20",
+      "task_label": "20",
+      "task_title": "End-to-end + crash recovery tests",
+      "session_id": "opencode:ses_1458aa770ffePqE4ip81RsOmNR",
+      "agent": "Sisyphus-Junior",
+      "category": "deep",
+      "updated_at": "2026-06-12T06:20:06.478Z",
+      "started_at": "2026-06-12T06:18:40.751Z",
+      "status": "completed",
+      "ended_at": "2026-06-12T06:20:06.478Z",
+      "elapsed_ms": 85727
     }
   },
   "agent": "atlas"

.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