Add Python 3.13 support

[igerber]

Summary

• Add Python 3.13 to build matrices (Linux, macOS, Windows wheels in publish.yml)
• Add Python 3.13 to CI test matrix (rust-test.yml): python-tests now runs 3.11 + 3.13 on all 3 OSes; python-fallback switches from 3.11 to 3.13
• Cap requires-python at >=3.9,<3.14 to prevent broken installs on untested versions
• Add Programming Language :: Python :: 3.13 classifier to pyproject.toml
• Update README requirements to show supported range Python 3.9 - 3.13
• Add "Keep in sync" comments to manual pip install lines in CI workflow

Methodology references (required if estimator / math changes)

• N/A - no methodology changes (CI/build/docs only)

Validation

• Tests added/updated: No test changes needed — existing 1285 tests pass on current Python
• CI matrix expansion will validate 3.13 compatibility on push

Security / privacy

• Confirm no secrets/PII in this PR: Yes

---

Generated with Claude Code

[github-actions]

Overall assessment: ✅ Looks good

Executive summary

• CI/build matrices and package metadata now include Python 3.13; README range updated accordingly.
• No estimator, math, weighting, or inference changes detected in the diff.
• Potential release gap: Linux wheel build will silently skip 3.13 if the manylinux image doesn’t contain cp313.
• Pure-Python fallback is now only validated on 3.13, leaving the min-supported 3.9 path untested.

Methodology
No estimator/math/SE/assumption changes in this PR; Methodology Registry and source-paper cross-checks are not applicable.

Code Quality
No issues found.

Performance
No issues found.

Maintainability

• P2. Impact: If quay.io/pypa/manylinux_2_28_x86_64 lacks cp313, Linux 3.13 wheels are silently skipped while metadata/README claim 3.13 support, potentially surprising Linux users at install time. Fix: Fail the build when cp313 is missing or pin/update the manylinux image to one known to include 3.13, and/or explicitly validate that cp313 exists before release. Location: .github/workflows/publish.yml:L22-L30.

Tech Debt
No issues found.

Security
No issues found.

Documentation/Tests

• P3. Impact: Pure-Python fallback is now only exercised on 3.13, so regressions specific to the minimum supported version (3.9) could slip. Fix: Run the fallback job on 3.9 (or add a small matrix covering 3.9 and 3.13). Location: .github/workflows/rust-test.yml:L136-L157.