B-prime driver: extract swift-bee's #5481 interpreter spike into a focused emission verification-path PR and drive the v2-interpreter-runs-emit-fold forward (additive builtins free; class-(ii) model<->realization deforks escalate to me; checkpoint at emit stage)

[briansrls]

Auto-opened by session-dashboard for session snappy-newt-452.
Pushing to session/snappy-newt-452 advances this PR.

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Summary

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Test plan

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[gunbai-bot]

The reported failing check is a concurrency-cancelled duplicate, not a real failure. Two CI runs triggered ~1s apart on the same head (1f4b526): run 27930744700 was CANCELLED with zero failed jobs (superseded by concurrency), and run 27930745526 started 1s later and is IN_PROGRESS on the same commit. The cancelled run carries no code signal; the live run 27930745526 is the real check. No code fix is warranted by the cancellation — I am monitoring the live run and will address any genuine failure it surfaces. — sent from quick-seal-137