Add snapshot_settings context manager with filter support

[MatthewMckee4]

Summary

• Add karva.snapshot_settings(filters=[(regex, replacement), ...]) context manager for filtering non-deterministic content from snapshots
• Filters are applied sequentially to serialized output before comparison/storage
• Nested settings accumulate filters from outer to inner scope
• Invalid regex patterns produce clear ValueError messages

Test plan

• Basic filter replaces date pattern with [date]
• Multiple filters applied sequentially (date + UUID)
• No-match filter leaves output unchanged
• Invalid regex produces clear ValueError
• Nested settings apply both outer and inner filters
• Filters work with inline snapshots
• Filters work with --snapshot-update mode
• Empty filters list behaves same as no settings
• Full test suite passes (just test — 558 tests)
• All lints pass (prek run -a)

🤖 Generated with Claude Code

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing add-snapshot-settings-filters (b7333ab) with main (3cbefa5)}

⚠️ Unknown Walltime execution environment detected

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Summary

✅ 1 untouched benchmark