Record a loop-body signal once when delivery races the wait

[alexanderguy]

Summary

• A SignalReceived whose id is already on the run is not appended again, on the buffered commit path and on the immediate commit path. A duplicate that arrives after the run is terminal still fails in applyEvent before it is buffered. The immediate path still flushes events already in the buffer.
• A loop body on a host signal channel receives one delivery after the relay wait is subscribed, and one delivery while that wait is still buffered. The parent log and the iteration log each hold one SignalReceived. The run completes converged, and the escalation handler does not run.

Verification

• make all exits 0 on this branch.
• Unit tests report 8768 pass and 0 fail. Workflow tests report 35 pass and 0 fail.

Closes INTR-568

[alexanderguy]

Self-review (code-review skill) returned clean.

[linear]

INTR-568