[bbopen]

Scope

The frozen revision-2 union policy rejects a contract when one wire value can decode as different types. This draft adds a pairwise check during pure callable compilation.

The regression covers bytes markers, legacy ndarray and Torch markers, nested unions, a located diagnostic, and an unknown generated return. It keeps disjoint records and int | float supported. The guard treats decoder-required fields separately from fields a legacy marker can omit.

Evidence and gate

Independent source review accepted b11dae6. Full CI run 35505214309 passed at that head. The final 9e87968 commit changes only Prettier formatting, and the pinned formatter passes. An earlier run at 88859e2 failed one same-run data-plane overhead check (3.74x versus 3.5x); the later full run passed, and this branch changes no data-plane source.

This remains a draft core correction for #345. It does not include revision-3 exact integer work. A separate merge candidate combines both histories for independent conflict review and combined exact-head CI. No merge, release, or issue closure is requested.

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[bbopen]

Integrated through reviewed implementation PR #362, merged as 43f111b43fbb0ce31dcb59f8fb489e8dadf3f191. This branch head is an ancestor of that merged commit. Final combined release candidate aa5497f867b36a76670afedbe9b34a6712a322e3 passed full CI 35508107651. Closing this component PR because its work is already on main; release publication is tracked by #354.