chore: migrate type checker from mypy to ty

[pesap]

Summary

• Replace mypy with ty (Astral's Rust-based type checker) in dev dependencies
• Replace [tool.mypy] configuration with [tool.ty] sections in pyproject.toml
• Update pre-commit hook to run ty check instead of mypy
• Add a dedicated ty CI job with --output-format github for inline PR annotations
• Gate pytest and benchmarks jobs on both prek and ty passing

Why

ty is significantly faster than mypy (10-100x, written in Rust) and is actively maintained by Astral (the same team behind ruff and uv). The codebase passes clean under ty with zero changes to source code.

Notes

• ty is still beta (0.0.x), so the minimum version is pinned at >=0.0.23
• ty respects # type: ignore comments by default, so existing type suppression continues to work
• .ty_cache/ added to .gitignore; mypy entries kept for stale cache cleanup

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.63%. Comparing base (67ca845) to head (3c65987).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #106   +/-   ##
=======================================
  Coverage   95.63%   95.63%           
=======================================
  Files           8        8           
  Lines        1719     1719           
=======================================
  Hits         1644     1644           
  Misses         75       75           

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