autotagging: make Change polymorphic

[snejus]

Fixes #6688

This change simplifies import preview rendering by routing both album and track matches through a single show_change() flow.

Architectural impact

• Introduces a shared Change.from_match() factory that selects AlbumChange or TrackChange based on the match type.
• Moves preview rendering behind polymorphic show() methods, so each change type owns its own display behavior.
• Removes separate track-specific preview handling like show_item_change() and the related branching in session.py.

High-level effect

• Centralizes change-preview logic in beets/ui/commands/import_/display.py.
• Makes session.py simpler by using one preview entry point for both albums and tracks.
• Reduces duplicate logic and makes future display changes easier to implement consistently.

In short: this refactor does not change the import flow itself, but it makes the change-display architecture more uniform, easier to follow, and easier to extend.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.60%. Comparing base (da82f8f) to head (d26e8f2).

Files with missing lines	Patch %	Lines
beets/ui/commands/import_/display.py	80.00%	5 Missing ⚠️
beets/ui/commands/import_/session.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           introduce-source    #6689   +/-   ##
=================================================
  Coverage             72.60%   72.60%           
=================================================
  Files                   163      163           
  Lines                 20811    20812    +1     
  Branches               3288     3287    -1     
=================================================
+ Hits                  15109    15110    +1     
  Misses                 4978     4978           
  Partials                724      724           

Files with missing lines	Coverage Δ
beets/ui/commands/import_/session.py	57.91% <66.66%> (-0.38%)	⬇️
beets/ui/commands/import_/display.py	81.86% <80.00%> (+0.27%)	⬆️

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[Copilot]

Pull request overview

grug see PR make import preview use one show_change() path for album and track. This fits import UI by moving display choice into Change.from_match() and show() methods.

Changes:

• Rename shared preview object to Change and add factory for album vs track match.
• Move album tracklist display methods onto AlbumChange.
• Simplify import session calls to always use show_change().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
beets/ui/commands/import_/display.py	Adds polymorphic change display flow and album/track subclasses.
beets/ui/commands/import_/session.py	Removes track-specific preview branch and calls shared display entry point.