Add %tunique{} path template function for track disambiguation within albums

[arsaboo]

Adds a new %tunique{} path template function (analogous to %aunique{} for albums and %sunique{} for singletons) that disambiguates tracks within the same album that would otherwise produce identical filenames (e.g., two tracks with the same title on the same album or across different discs). This follows the structure and logic used in aunique or sunique.

Fixes #5950

Default configuration:

tunique:
    keys: title
    disambiguators: track disc artist
    bracket: '[]'

The function first finds all sibling items (same album_id) with matching key values (default: same title). If duplicates exist, it iterates through the disambiguator fields looking for one that uniquely separates all duplicates. If no field suffices, the stable database ID is used as a fallback. For items without a duplicate, no suffix is added.

This solves the problem where beet move would non-deterministically append .1/.2 suffixes via unique_path() for duplicate-titled tracks, which would change every time the command was re-run.

Example usage in path config:

paths:
    default: $albumartist/$album/$album ($year) - $title%tunique{}

For an album with two identically-titled tracks:

- Artist/Album/Album (1993) - Common Title [07].mp3
- Artist/Album/Album (1993) - Common Title [11].mp3

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a new %tunique{} path template and configuration to disambiguate tracks with identical titles within the same album.

Changes:

• Introduces tmpl_tunique and a query override hook in _tmpl_unique to scope ambiguity checks to an album.
• Adds defaults for tunique in config_default.yaml and documents the new template/config.
• Adds a new test suite covering %tunique{} behavior and edge cases.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/test_library.py	Adds %tunique{} test coverage for album-scoped track title collisions and formatting options.
docs/reference/pathformat.rst	Documents the new %tunique{} template and its default identifiers/disambiguators.
docs/reference/config.rst	Documents new tunique config keys and defaults.
docs/changelog.rst	Adds changelog entry announcing %tunique{}.
beets/library/models.py	Implements tmpl_tunique and extends _tmpl_unique with an optional pre-built query.
beets/config_default.yaml	Adds default configuration for tunique.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.62%. Comparing base (4240594) to head (eaa9ac3).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beets/library/models.py	80.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6722   +/-   ##
=======================================
  Coverage   75.61%   75.62%           
=======================================
  Files         163      163           
  Lines       21323    21338   +15     
  Branches     3363     3367    +4     
=======================================
+ Hits        16124    16137   +13     
- Misses       4406     4407    +1     
- Partials      793      794    +1     

Files with missing lines	Coverage Δ
beets/library/models.py	87.02% <80.00%> (-0.32%)	⬇️

... and 1 file with indirect coverage changes

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[snejus]

I think it would be best to wait until #6825 is merged as I added types to these methods here.