- requires-python >=3.11 → >=3.10 (no 3.11 features used)
- build_hooks.py and provider_coverage.py: version-gated tomllib import
  with tomli backport for Python 3.10
- Added tomli>=2.0 to build-system.requires and runtime dependencies
  (conditional on python_version < 3.11)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>