Add Python 3.15 wheel and test coverage

[patrick91]

Summary

• build CPython 3.15 and free-threaded 3.15t wheels on every supported platform
• test both 3.15 variants and advertise Python 3.15 support
• update to PyO3 0.29.1, Maturin 1.14.1, and patrick91/wheels-ci-action@v1

Validation

• cargo fmt --check
• cargo check --locked
• uv lock --check
• ruff check tests
• ruff format --diff tests
• mypy tests
• workflow YAML parse and git diff --check
• local CPython 3.15 and 3.15t extension builds completed

Local execution is blocked by a mis-aligned LINKEDIT string pool error from the current macOS/Xcode/Rust linker combination. The same error reproduces on the existing Python 3.14 target, so the hosted Linux 3.15 and 3.15t jobs are the runnable compatibility check.

[codspeed-hq]

Merging this PR will not alter performance

✅ 21 untouched benchmarks

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_{Comparing patrick91:agent/python-315-support (acd426e) with main (2b34bf5)}