build: replace hatchling with the uv_build backend

[vdusek]

Switches the build backend from hatchling to uv_build, replacing the [tool.hatch.build.targets.*] config with [tool.uv.build-backend]. Only CHANGELOG.md and CONTRIBUTING.md need an explicit source-include - the module, pyproject.toml, README, and LICENSE are in the sdist by default.

License metadata moves to PEP 639: license = "Apache-2.0" plus license-files = ["LICENSE"]. That part is required, not cosmetic: uv_build ships LICENSE inside the wheel only when it is declared via license-files, and uv rejects license-files combined with the legacy license = { file = ... } table.

The License :: OSI Approved :: Apache Software License classifier is dropped because PEP 639 deprecates license classifiers in favor of License-Expression, which now carries the same information ("Using license classifier in the Classifier Core Metadata field is deprecated and replaced by the more precise License-Expression field"). See also the core metadata spec. uv warns about the classifier on every build.

Verified by building sdist + wheel before and after: the wheel contents are identical, and the metadata differs only in License-Expression: Apache-2.0 replacing the inlined license text. The sdist drops .gitignore and gains pyproject.toml.orig (uv 0.12 writes a TOML 1.0-compatible pyproject.toml into the sdist and keeps the original alongside it). uv.lock is unchanged.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.62%. Comparing base (b49fb35) to head (3d377de).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #980   +/-   ##
=======================================
  Coverage   94.62%   94.62%           
=======================================
  Files          58       58           
  Lines        5248     5248           
=======================================
  Hits         4966     4966           
  Misses        282      282           

Flag	Coverage Δ
integration	92.26% <ø> (ø)
unit	83.89% <ø> (ø)

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