lastgenre: Genre spelling normalization (aliases)

[JOJ0]

Description

This PR introduces a regex-based normalization (alias) system to unify variant genre tags and improves the plugin's documentation.

The normalization feature uses an ordered list of regular expression aliases to map variant spellings or synonyms to a single canonical name. The mapping keys act as re.Match.expand() templates, supporting \g<N> back-references to regex capture groups:

lastgenre:
    aliases:
        # Maps 'hip-hop' -> 'hip hop' using back-references 
        # (regex full-match)
        \g<1> hop:
            - (hip|trip|jazz)y?[ /-]*hop
        
        # Static normalization examples
        drum and bass:
            - dnb
            - d&b

• Where?: Integrated normalize_genre before is_ignored in both the LastFmClient (for clean fetching/caching) and the core _resolve_genres loop (for uniform processing of existing file tags).
• Additional Documentation Changes:
  • Added a Choosing the Right Tool guide. This is for endusers but note that it might even be a primary starting point for maintainer review.
  • Added non-whitelist mode details to the Canonicalization chapter.

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.96%. Comparing base (c5a14d3) to head (f4b9436).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/lastgenre/__init__.py	90.90%	1 Missing and 2 partials ⚠️
beetsplug/lastgenre/client.py	66.66%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           lastgenre_forbidden    #6466      +/-   ##
=======================================================
+ Coverage                69.91%   69.96%   +0.04%     
=======================================================
  Files                      146      146              
  Lines                    18575    18642      +67     
  Branches                  3027     3052      +25     
=======================================================
+ Hits                     12987    13042      +55     
  Misses                    4957     4957              
- Partials                   631      643      +12     

Files with missing lines	Coverage Δ
beetsplug/lastgenre/utils.py	100.00% <100.00%> (ø)
beetsplug/lastgenre/__init__.py	83.22% <90.90%> (+0.42%)	⬆️
beetsplug/lastgenre/client.py	67.85% <66.66%> (+12.75%)	⬆️

... and 11 files with indirect coverage changes

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[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[JOJ0]

This is closely related to #6449 and might make sense to have a look when reviewing the former @snejus Many thanks in advance! 🙌

[JOJ0]

Reviewers: This new chapter is a good starting point for reviewing and might help thinking through where and when things should happen in the plugin and why multiple filtering features make sense for this plugin!