fix(wal): checkpoint 重写改为临时文件+原子 rename，堵住崩溃丢数据窗口

[NeverENG]

问题

doRewrite（#157 引入）就地 Truncate(0) → 重写 → Sync，三步非原子。若在 Truncate 之后、重写 fsync 之前进程崩溃，WAL 被清空/残缺——而 checkpoint 快照的 active+dirty 此刻尚无 SSTable 副本（正因如此才在快照里），会静默丢失已 ack 的写，违背"Append 返回即已 fsync 落盘"的契约。

修复

标准原子替换：records 写满并 fsync 到 wal.log.tmp → os.Rename 覆盖（POSIX 原子）→ fsync 父目录让 rename 持久。恢复时只会看到完整旧 WAL 或完整新 WAL，绝不残缺。仍只由 flushLoop 执行，维持文件唯一写者不变式。

验证

TestWALRewriteAtomicReplace：陈旧 .tmp 残留不干扰恢复；重写后恰只剩新记录（含墓碑/空值边界）、可继续 Append、重开仍完整。全量 -race 通过。

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Review details

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between 3d935df and 1e04a6c.

📒 Files selected for processing (2)

• storage/wal.go
• storage/wal_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/wal-atomic-checkpoint

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