Add --watch flag to karva test

[MatthewMckee4]

Summary

• Add --watch flag to karva test that re-runs tests automatically when .py files change
• Uses notify-debouncer-mini crate for file watching with 200ms debounce
• Uses crossbeam_channel::select! to wait on both file change events and Ctrl+C shutdown signal
• On file change: clears screen, prints changed files, re-runs all tests
• Errors with a clear message when combined with --dry-run

Test plan

• test_watch_and_dry_run_conflict — verifies error when both flags used
• test_watch_runs_and_can_be_killed (unix) — spawns watch mode, waits, kills, verifies tests ran
• just test — all 640 tests pass
• uvx prek run -a — all pre-commit checks pass

Closes #477

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing add-watch-mode (a031875) with main (003ed9a)}