create: implement "file changed while backup" detection on Windows, fixes #9382

[mr-raj12]

Summary

• The "file changed while being backed up" detection (fstat2 check) was entirely skipped on Windows via if not is_win32: # TODO for win32 since b3751b1 (March 2019, should print a warning if file changed during backup #1750)
• On Windows, st_ctime is file creation time (not inode change time), so the default ctime-based detection was meaningless - but the mtime comparison path (added in b27df15, July 2025) works correctly on Windows since st_mtime updates
  on writes
• Fix: remove the is_win32 guard so the fstat2 + comparison logic runs on all platforms, and default --files-changed to mtime on Windows instead of ctime
• If a user explicitly passes --files-changed=ctime on Windows, borg now warns and falls back to mtime instead of silently disabling detection

Fixes #9382

Changes

File	Change
src/borg/archive.py	Remove if not is_win32 guard at fstat2 check; default files_changed to "mtime" on win32
src/borg/archiver/create_cmd.py	Platform-aware argparse default; warn + fallback on --files-changed=ctime on Windows; update help text and docs
src/borg/testsuite/archiver/create_cmd_test.py	Add test_files_changed_mtime — runs on all platforms including Windows

Related issues: #1750, #7193, #8730

Checklist

• PR is against master
• New code has tests
• Commit message references related issue
• Code formatted with black

[codecov]

Codecov Report

❌ Patch coverage is 29.41176% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.43%. Comparing base (cace06f) to head (4f2480c).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archive.py	35.71%	4 Missing and 5 partials ⚠️
src/borg/archiver/create_cmd.py	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9384      +/-   ##
==========================================
- Coverage   76.44%   76.43%   -0.01%     
==========================================
  Files          85       85              
  Lines       14799    14802       +3     
  Branches     2212     2212              
==========================================
+ Hits        11313    11314       +1     
- Misses       2808     2809       +1     
- Partials      678      679       +1     

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[ThomasWaldmann]

is_win32 is true for native windows (msys2/mingw).

what is the situation for cygwin (is_cygwin)?

[mr-raj12]

Cygwin has is_win32 = False, so none of these changes affect it and it was already running the fstat2 check with ctime before, and still does. Cygwin maps st_ctime to NTFS ChangeTime (POSIX semantics), not CreationTime, so ctime works fine there.