Fix false repo relocation warning on macOS due to NFC/NFD path differences (#2913)

[ThomasWaldmann]

Fixes #2913.

Problem

On macOS the filesystem returns paths in NFD (decomposed) unicode form, while a path typed as a command line argument is usually in NFC (composed) form. These render identically but are byte-different. So accessing a repo whose path contains non-ASCII characters via a relative path / . / $(pwd) after cd made borg believe the repository had been relocated:

cd /tmp && mkdir empty
borg init -e none tëst
borg create tëst::test1 /tmp/empty
cd tëst
borg create .::test2 /tmp/empty
>> Warning: The repository at location /private/tmp/tëst was previously located at /private/tmp/tëst
>> Do you want to continue? [yN]

The two locations are visually identical because one is NFC and the other NFD.

Fix

• Add helpers.normalize_local_path() — normalizes to NFD on macOS (the form the filesystem itself uses, matching what patterns.normalize_path() already does), a no-op on other platforms.
• Run local file locations through it in Location.canonical_path().
• Normalize both sides of the comparison in SecurityManager.assert_location_matches(), so existing NFC-form security location files still match after upgrade (no one-time spurious prompt) and remote (rest/ssh/borgstore) URLs stay consistent (normalization is idempotent and applied symmetrically).

Scope / safety

• No-op on non-macOS platforms (is_darwin is fixed at import time).
• Only affects the repository location string used for the security/relocation check and its display — archived file paths are unaffected.

Tests

Added test_normalize_local_path and test_canonical_path_unicode_normalization (both platform-aware). Full parseformat + archiver checks suites pass locally.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.68%. Comparing base (7bfd489) to head (4a25abc).
⚠️ Report is 15 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9940      +/-   ##
==========================================
- Coverage   85.68%   85.68%   -0.01%     
==========================================
  Files          95       95              
  Lines       16794    16799       +5     
  Branches     2574     2574              
==========================================
+ Hits        14390    14394       +4     
- Misses       1668     1669       +1     
  Partials      736      736              

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