feat(wal,memtable): implement segment writer, block writer, and MemTable

- wal/block_writer.go: 32KB block buffer with padding and flush
- wal/segment_writer.go: WAL segment file with durable-ready protocol
- memtable/memtable.go: Arena+SkipList wrapper with publish/abort semantics
- Comprehensive tests for all modules, all pass with -race

.omo/plans/phase1-wal.md

@@ -823,7 +823,7 @@ Max Concurrent: 6 (Wave 1b)
 
   **Commit**: YES (group with Task 6)
 
-- [ ] 10. Segment writer (file write + block writer)
+- [x] 10. Segment writer (file write + block writer)
 
   **What to do**:
   - 创建 `wal/block_writer.go`：`BlockWriter` — 管理 32KB block 的 Physical Record 写入
@@ -1105,7 +1105,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES (with Task 12)
   - Message: `feat(wal): implement WAL writer with group commit`
 
-- [ ] 14. MemTable (wrap skiplist + arena + publish/abort)
+- [x] 14. MemTable (wrap skiplist + arena + publish/abort)
 
   **What to do**:
   - 创建 `memtable/memtable.go`：`MemTable` 结构体