Build stable-ABI wheels for Python 3.12+

[marco-mariotti]

Summary

• upgrade PyO3 and numpy-rust to 0.29
• enable PyO3's abi3-py312 stable ABI
• build one wheel per supported platform/architecture instead of one per Python minor version
• test the Linux x86_64 ABI3 wheel on Python 3.12, 3.13, and 3.14 before the publish job can run
• document the stable-ABI Python floor

Context

This revisits the stable-ABI proposal from #24 against the current repository.
The package now requires Python 3.12, the core algorithms live in
ruranges-core, macOS wheels are built natively, and package versions are
already synchronized explicitly. Accordingly, this uses abi3-py312, retains
the current versioning scheme, and updates the current release workflow rather
than applying #24 verbatim.

This PR intentionally does not close #24 automatically; its discussion can be
wrapped up separately.

Impact

The release set falls from eight wheels to four for the current two Python
versions and four platform/architecture combinations. The same four wheels
also cover Python 3.14 and future compatible CPython releases without adding a
new build per Python minor version.

Local matched-build testing found no meaningful performance or memory cost:

• 288 successful process benchmarks across all eight established operations,
  hg38 inputs from 1M through 100M, four implementations, and three repetitions
• ABI3/native geometric-mean runtime change: +0.11%
• ABI3/native geometric-mean peak-RSS change: -0.03%
• zero output-row parity mismatches
• median timing coefficient of variation: 0.73%, larger than the measured ABI difference

Validation

• built ruranges-0.2.7-cp312-abi3-macosx_11_0_arm64.whl locally with maturin 1.14.1 and Rust 1.96.0
• imported and exercised the built ABI3 extension through PyRanges1 1.4.3
• ran the full local benchmark matrix described above
• parsed the updated workflow as YAML and checked the patch with git diff --check