Add BenchmarkRebuildBlockedCache_Large and _XLarge to measure cache
rebuild performance in isolation.
Results show cache rebuild takes ~773ms at 10K issues and ~1.3s at 20K,
significantly slower than the ~50ms originally estimated in the issue.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>