Migrate to pyproject.toml and towncrier fragments

[MaxGhenis]

Summary

• Migrates from setup.py to pyproject.toml (hatchling build backend)
• Replaces changelog_entry.yaml with towncrier fragments (changelog.d/<branch>.<type>.md) to eliminate merge conflicts
• Each PR gets its own uniquely-named fragment file instead of all editing the same file
• Simplifies PR CI: lightweight check that a fragment exists (no Python install needed)
• Push workflow uses towncrier for version bumping and changelog generation

Fragment format

echo "Description." > changelog.d/<branch-name>.<type>.md
# Types: added (minor), changed (patch), fixed (patch), removed (minor), breaking (major)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.51%. Comparing base (58c2fca) to head (4ed827f).
⚠️ Report is 36 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
+ Coverage   81.14%   82.51%   +1.37%     
==========================================
  Files         197      202       +5     
  Lines       10228    10634     +406     
  Branches     1057     1069      +12     
==========================================
+ Hits         8299     8775     +476     
+ Misses       1640     1583      -57     
+ Partials      289      276      -13     

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