feat(memtable): implement SkipList with lock-free reads

- memtable/skiplist.go: heap-backed SkipList with atomic.Pointer next pointers
- Mutex-protected writes, lock-free reads with release/acquire ordering
- Copy-on-insert and copy-on-return for key/value safety
- Forward iterator that skips pending entries
- Comprehensive tests: ordered insert, overwrite, pending hidden, concurrent -race

.omo/plans/phase1-wal.md

@@ -694,7 +694,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES (with Task 8)
   - Message: `feat(memtable): implement Arena allocator and SkipList`
 
-- [ ] 8. SkipList (mutex write + lock-free read)
+- [x] 8. SkipList (mutex write + lock-free read)
 
   **What to do**:
   - 创建 `memtable/skiplist.go`：`SkipList` 结构体