docs: bind C++ corpus evidence

[zmaril]

Summary

• bind the completed 121,957-file C++ sweep to the exact corpus lock and grammar identity
• replace the obsolete 791-file libstdc++ stand-in description with the real 97-package Debian population
• record the measured grammar-gap, configuration and noise buckets
• document the one reproducible libclang crash that the new isolation path safely classified as indeterminate

Result

• 30,123 passing
• 15,601 verified grammar gaps
• 267 configuration-inherent failures
• 75,966 noise cases, including 75,725 indeterminate files
• 10 current corpus evidence records, 0 stale, 0 unbound

Validation

• cargo test --workspace
• treebank status --check
• actionlint
• git diff --check