feat(wal): implement batch codec, validation, and Arena allocator

- wal/batch.go: WalBatch encode/decode with FragmentCollector state machine
- wal/validate.go: ValidateBatchLimits with checked arithmetic
- memtable/arena.go: Arena allocator with 8-byte alignment and mutex
- Comprehensive tests for all modules, all pass with -race

.omo/plans/phase1-wal.md

@@ -555,7 +555,7 @@ Max Concurrent: 6 (Wave 1b)
 
   **Commit**: YES (group with Tasks 3, 4)
 
-- [ ] 6. WAL Batch encode/decode + fragment collector
+- [x] 6. WAL Batch encode/decode + fragment collector
 
   **What to do**:
   - 创建 `wal/batch.go`：`WalBatch` 结构体（Flags uint16, BaseSequence uint64, EntryCount uint32, EntriesSize uint32, Entries []byte）
@@ -633,7 +633,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES (group with Task 9)
   - Message: `feat(wal): implement WAL batch codec and resource validation`
 
-- [ ] 7. Arena allocator
+- [x] 7. Arena allocator
 
   **What to do**:
   - 创建 `memtable/arena.go`：`Arena` 结构体
@@ -759,7 +759,7 @@ Max Concurrent: 6 (Wave 1b)
 
   **Commit**: YES (with Task 7)
 
-- [ ] 9. WAL Batch resource validation
+- [x] 9. WAL Batch resource validation
 
   **What to do**:
   - 创建 `wal/validate.go`：`ValidateBatchLimits(entries []*WalEntry, cfg *config.WalConfig) error`