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 lookup/filtering) and the core _resolve_genres loop (for uniform processing of existing file tags).
• Updated default whitelist and genre tree files for canonical-name consistency, and curated the bundled aliases.yaml from the top 1,000 Last.fm tags to cover common inconsistencies without over-normalizing.
• 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.
• Additional unrelated changes:
  • Fixes a few variable name inconsistencies in the recently merged ignorelist features' tests.

[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 93.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.53%. Comparing base (da16db1) to head (000ff8f).

Files with missing lines	Patch %	Lines
beetsplug/lastgenre/client.py	70.00%	1 Missing and 2 partials ⚠️
beetsplug/lastgenre/__init__.py	97.22%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6466      +/-   ##
==========================================
+ Coverage   72.46%   72.53%   +0.06%     
==========================================
  Files         161      161              
  Lines       20719    20767      +48     
  Branches     3280     3295      +15     
==========================================
+ Hits        15014    15063      +49     
+ Misses       4979     4973       -6     
- Partials      726      731       +5     

Files with missing lines	Coverage Δ
beetsplug/lastgenre/utils.py	100.00% <100.00%> (ø)
beetsplug/lastgenre/__init__.py	83.16% <97.22%> (+1.48%)	⬆️
beetsplug/lastgenre/client.py	64.58% <70.00%> (+10.92%)	⬆️

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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!

[JOJ0]

Hi @snejus I commented inline (sorry it's pretty messy here already :-/) addressed all suggestions and I am happy with the result. Please have a (hopefully final) look :-)

Note to myself: I'll probably simply squash all the commits or maybe reset and recommit everything so at least changes not directly involved with this new feature stay separate!