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 72.20%. Comparing base (0ab47d3) to head (e232ff1).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3277   +/-   ##
=======================================
  Coverage   72.20%   72.20%           
=======================================
  Files          56       56           
  Lines        2425     2425           
  Branches      423      423           
=======================================
  Hits         1751     1751           
  Misses        595      595           
  Partials       79       79           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.