docs: Qwen3.5 graft Report 13 cross-attention results

[mvillmow]

Summary

• documents the sealed Qwen3.5 graft Report 13 causal cross-attention experiment
• reports every evaluated checkpoint from 1M through 250M tokens
• binds conclusions to immutable training and evaluation receipt hashes

Result

The 100M checkpoint improved the combined MMLU-Pro + BBH score by 0.005789 over baseline, but the gain did not persist through 250M.

Validation

• git diff --check
• verified all checkpoint/evaluation completion receipts and protocol identity
• MkDocs/pre-commit tooling is unavailable in the current environment

[github-actions]

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