Adds a benchmark comparing monolithic PCH vs chained PCH (one link per
Key findings on 70 C++ stdlib headers:
- Monolithic full build: ~1300ms
- Chained full build: ~2600ms (2x, expected serialization overhead)
- Incremental append-one-link: ~37ms vs ~1300ms monolithic rebuild (36x speedup)
- All 70 chain links compile and verify successfully
Also documents that PrecompiledPreambleBytes bound must be 0 for chained
PCH (each link is a separate file, previous PCH doesn't cover current file).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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