feat(wal): implement segment rotation, recovery scanner, and record parser

- wal/segment_manager.go: segment lifecycle with rotation at batch boundaries
- wal/scanner.go: segment discovery, ordering, and continuity validation
- wal/record_parser.go: block-level physical record parsing with tail corruption detection
- Comprehensive tests for all modules, all pass with -race

.omo/plans/phase1-wal.md

@@ -891,7 +891,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES (with Task 11)
   - Message: `feat(wal): implement segment writer with rotation`
 
-- [ ] 11. Segment rotation + durable-ready protocol
+- [x] 11. Segment rotation + durable-ready protocol
 
   **What to do**:
   - 创建 `wal/segment_manager.go`：管理 segment 的轮转和生命周期
@@ -1180,7 +1180,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES
   - Message: `feat(memtable): integrate arena+skiplist with publish/abort`
 
-- [ ] 15. WAL recovery: scanner + record parser
+- [x] 15. WAL recovery: scanner + record parser
 
   **What to do**:
   - 创建 `wal/scanner.go`：`SegmentScanner` — 从 WAL 目录扫描 segment 文件