ci: optimize Python validation

[rlippmann]

Summary

Optimize Python CI by running Ruff and mypy once on Python 3.11, while retaining pytest coverage across Python 3.11–3.14. Enable uv caching and locked dependency synchronization.

Why

Avoid redundant static checks in every Python version matrix job while preserving runtime compatibility coverage.

Example metadata

• Enforcement point: CI validation workflow
• Domain: repository maintenance
• Technology/runtime: GitHub Actions, uv, Python
• Example type: repository maintenance
• Does this use directive drafter? should normally be no: No
• Does this derive state from model output? must be no for primary examples: No

Checklist

• Repository scaffolding / governance change
• One primary enforcement point
• Explicit authoritative state
• No model-derived state mutation
• Adversarial stub or equivalent test
• Observable runtime behavior change
• Domain vocabulary is natural
• Framework is secondary to the enforcement point
• Docs updated
• Tests/smoke checks added where practical

Notes

Validation passed: YAML parse, diff check, locked dependency sync, Ruff, Ruff format, mypy, and pytest (212 passed, 6 skipped).

Only .github/workflows/ci.yml changed.