fix: proper perspective aspect ratio preservation

[gselzer]

Turns out that perspective matrices weren't actually having their aspect ratios preserved. This PR documents what the (newly-renamed) letterbox parameter does, and adds in a test ensuring it behaves as expected.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.08%. Comparing base (d5236c4) to head (6a5dda8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scenex/utils/projections.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
- Coverage   88.09%   88.08%   -0.01%     
==========================================
  Files          64       64              
  Lines        3174     3182       +8     
==========================================
+ Hits         2796     2803       +7     
- Misses        378      379       +1     

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[Copilot]

Pull request overview

This PR fixes aspect-ratio preservation for zoom_to_fit(..., type="perspective") by introducing/renaming the option to letterbox, documenting its behavior, and adding regression tests to ensure non-square viewports no longer cause anisotropic distortion.

Changes:

• Renamed preserve_aspect_ratio to letterbox and updated internal logic to apply letterboxing for both orthographic and perspective projections.
• Added a new perspective letterbox test (and renamed the existing orthographic aspect test) to validate correct NDC mapping for wide/tall canvases.
• Updated scenex.util.show() to use the new letterbox=True parameter.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/scenex/utils/projections.py	Implements letterbox behavior (including perspective aspect correction), updates docstring, and adds _aspect_ratio() helper.
tests/utils/test_projections.py	Renames orthographic aspect test and adds a new regression test for perspective letterboxing.
src/scenex/util.py	Updates show() to call zoom_to_fit(..., letterbox=True).

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