docs: make benchmark validation reproducible

#1

README-only: explicit train split, exact seven-case validation matrix pinned to immutable fixture/verifier commits, and fail-closed scope/limitations. No fixture or license changes.

Validated PR head: README-only; card/config parse; direct load 7/7 rows; all 7 fixture outcomes reproduced with pinned verifier; fixture byte-diff invariants pass.

betterwithage changed pull request status to merged

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