Replace Likelies with a frozen dataclass

[aribradshaw]

Description

Fixes #6920.

Replace the fixed-schema Likelies AttrDict with a frozen dataclass and construct album and singleton metadata explicitly. Album import metadata is converted to a detached dictionary at the task boundary, while singleton ASIS and RETAG tasks continue to preserve flexible item fields.

The refactor makes the fixed fields and immutability explicit without leaking a mutable internal metadata object to importer consumers.

To Do

• Documentation. No public interface or command-line behavior changed.
• Changelog.
• Tests.

Validation

• ruff check on all changed Python files
• ruff format --check on all changed Python files
• 10 focused regression and plurality tests passed
• A broader related-file run produced 219 passes and 10 skips. Three existing Windows-specific extension-recognition tests failed independently of this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.07%. Comparing base (5406ed7) to head (c54703a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6934   +/-   ##
=======================================
  Coverage   76.06%   76.07%           
=======================================
  Files         163      163           
  Lines       21146    21152    +6     
  Branches     3334     3335    +1     
=======================================
+ Hits        16085    16091    +6     
  Misses       4273     4273           
  Partials      788      788           

Files with missing lines	Coverage Δ
beets/autotag/source.py	100.00% <ø> (ø)
beets/importer/tasks.py	91.07% <100.00%> (+0.05%)	⬆️
beets/util/__init__.py	81.75% <100.00%> (+0.06%)	⬆️

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.