Startup I/O: LogFileSink scans the log directory ~5x at boot; per-line sync appends

[LeeKuang666]

main.js:2470-2475 constructs LogFileSink then calls enforceDirectoryCap() and purgeOlderThan(7); each internally re-walks ownedFiles() and re-measures directoryBytes (log-files-Bfo6ODqx.js:69,:90-96,:112). Additionally every log line opens/writes/closes the file (:106 comment "files are closed after every append").

Suggestion: take one ownedFiles() snapshot shared by constructor/cap/purge; move trimming off the critical path; keep a persistent append handle.

[JunkaiWang-TheoPhy]

启动目录扫描的观察基本成立，#605 也已经针对扫描次数加入了实现和回归测试。

不过 #605 当前混入了无关的 CONTRIBUTING 文档 commit。建议先清理分支，只保留 LogFileSink 的扫描优化及其测试。每行 append 的同步关闭文件属于已有的 crash-safety 取舍，除非有基准证明它是 release-blocking 问题，否则建议另开性能 follow-up，不要和启动扫描一起合并。