fix(pre-oss): baseline SHA validated as mainline ancestor

[matdev83]

Tip-equality for Merged-Main Baseline SHA self-invalidates on every future main commit (treadmill: #584 green at creation, red after its own merge). This validates baseline as existing commit + ancestor of HEAD, same as Implementation SHA. Keeps exact vocab, 6-cell rows, row-level mandatory checks, anchored metadata, SHA-identity negatives.

1 Go file (test only).

[coderabbitai]

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