fix: prevent sparse gaps after cross-process log rotation

[Windsland52]

Summary

• keep every long-lived logger stream in append mode
• truncate a rotated log with a separate short-lived stream before reopening it
• prevent stale file offsets from creating large NUL-filled gaps when multiple processes share a log directory

Cause

After rotation, Logger previously reopened maafw.log with a long-lived truncating stream. If another process later rotated or truncated the same file, that stream retained its old file offset. Its next write could therefore create a large zero-filled gap, which then caused repeated oversized backups.

Validation

• compiled the modified Logger.cpp with MSVC in C++20 mode
• ran a two-process regression scenario: seed a 17 MiB log, rotate it, truncate the active log from a child process, then write again from the original logger
• resulting active log contained no NUL bytes (size=92, has_zero=false)
• clang-format --dry-run --Werror passed
• git diff --check passed

Sourcery 总结

通过将截断操作与长生命周期的日志写入分离，防止跨进程轮换后出现稀疏的日志空洞。

Bug 修复：

• 防止跨进程日志轮换在活动日志文件中创建稀疏的、填充 NUL 的空洞。

改进：

• 让长生命周期的日志记录器流保持追加模式，同时在重新打开期间通过短生命周期的流执行截断。

Original summary in English

Summary by Sourcery

Prevent sparse log gaps after cross-process rotation by separating truncation from long-lived log writing.

Bug Fixes:

• Prevent cross-process log rotation from creating sparse, NUL-filled gaps in active log files.

Enhancements:

• Keep long-lived logger streams in append mode while performing truncation through a short-lived stream during reopening.

[sourcery-ai]

嗨——我已经审阅了你的更改，看起来很棒！

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Original comment in English

Hey - I've reviewed your changes and they look great!

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[Windsland52]

cc @MistEO