feat: add RollingFileAppender::flush for durable log writes

[victoriacampbellmake]

Summary

Adds RollingFileAppender::flush() so callers can push OS/libc file buffers to disk without rolling or truncating logs (#233).

• util::File::flush() uses _commit on Windows and fsync on POSIX
• RollingFileAppender::flush() takes the appender lock and flushes the open log file
• Documented under RollingFileAppender in the README

This matches the backup / cross-process visibility use case discussed in the issue: flush durable bytes without calling rollLogFiles().

Test plan

• Log a few lines, call flush(), confirm another process sees the latest content
• Confirm flush() does not rename/remove log files
• Existing rolling behavior unchanged when flush() is unused

Closes #233