Add karva snapshot prune command

[MatthewMckee4]

Summary

• Add karva snapshot prune subcommand that detects and removes .snap files whose source test function no longer exists
• Uses static analysis (no Python/PyO3 needed): parses each snapshot's source metadata field to extract the test file and function name, then checks if they still exist on disk
• Supports --dry-run flag to preview which snapshots would be removed without deleting them
• Supports optional path filtering to scope pruning to specific directories or files
• Handles parametrized snapshots, numbered snapshots, inline snapshots, and class-prefixed test methods

Test plan

• 12 unit tests in karva_snapshot covering parse_source, base_function_name, find_snapshots, find_unreferenced_snapshots, and remove_snapshot
• 7 integration tests covering: basic prune, dry-run, nothing to prune, test file deleted, parametrized snapshots, path filtering, no snapshots directory
• just test — all 545 tests pass
• prek run -a — all pre-commit checks pass

[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-snapshot-prune-command (89cd92f) with main (6add357)}