Function deployStageGuards

Deploy every guard for a stage being entered (idempotent upsert), but only while the instance is still committed to that stage. A deploy that lost the race to a newer transition would otherwise re-lock a stage the instance already left — a stale lock the newer transition's retract can't see. A vanished instance (undefined) likewise no-ops: undefined !== stageName.

An aborted instance also no-ops: it parks on its stage forever, so a stale deploy landing after abort's retract would re-lock the subjects with no remaining path to delete them. The gate is abortedAt, not completedAt — a normal move into a terminal stage stamps completedAt in the same persist and must still deploy that stage's guards.