platform: implement Windows-native SyncFile using FILE_FLAG_WRITE_THROUGH

[mr-raj12]

On Windows, SyncFile currently uses the generic base implementation (src/borg/platform/base.py), which relies on os.fsync(). This does not guarantee data durability on Windows — os.fsync() maps to FlushFileBuffers(), which only flushes the OS cache to the drive's write cache, not necessarily to persistent storage.

The base class already has a TODO (line 154):

A Windows implementation should use CreateFile with FILE_FLAG_WRITE_THROUGH.

Current state

Platform	SyncFile	Data durability mechanism
Linux	Custom (linux.pyx)	sync_file_range + fdatasync
macOS	Base + custom fdatasync (darwin.pyx)	F_FULLFSYNC (PR #9385)
Windows	Base fallback	os.fsync() → FlushFileBuffers() — no write-through guarantee

Proposed solution

Implement a Windows-native SyncFile in src/borg/platform/windows.pyx that:

1. Opens the file using win32file.CreateFile (or ctypes/msvcrt) with FILE_FLAG_WRITE_THROUGH to bypass the drive write cache
2. Overrides sync() to use FlushFileBuffers() (already what os.fsync() does, but combined with write-through flag it provides stronger guarantees)
3. Optionally implements fdatasync() and sync_dir() for Windows in the platform module

Why this matters

Without FILE_FLAG_WRITE_THROUGH, writes on Windows may sit in the drive's volatile write cache. A power failure or crash can lose data that borg considers durably written, potentially corrupting the repository.

This is the Windows equivalent of the macOS F_FULLFSYNC fix (#9383 / PR #9385).

References

• TODO in src/borg/platform/base.py:154
• macOS F_FULLFSYNC: platform: use F_FULLSYNC on macOS for SyncFile data durability #9383 / PR platform: use F_FULLSYNC on macOS for SyncFile data durability, fixes #9383 #9385
• Linux sync_file_range: src/borg/platform/linux.pyx
• Windows FILE_FLAG_WRITE_THROUGH: Microsoft docs