perf: reduce mutex contention with early checks

Doug Barrettrequested to merge
perf/optimize-mutex-contention into master

Summary

Add lock-free early checks before acquiring mutex in Registry methods to reduce contention in high-throughput logging scenarios.

Changes

  • record_offense: Check if offense already exists before acquiring lock. Most repeated offenses will exit early without lock contention.
  • check_for_removed_offenses: Check if baseline exists before acquiring lock. Most callsites won't have a baseline, avoiding unnecessary locking.

Uses double-checked locking pattern to ensure thread safety while reducing lock contention.

Expected Performance Improvement

  • Significant reduction in mutex contention for repeated offenses
  • Most log calls with deprecated fields hit the same offenses repeatedly, so the early check avoids acquiring the lock entirely
  • Most callsites don't have a baseline, so check_for_removed_offenses returns early without locking

Relates-to: #60 (closed)

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