Record that a source was checked for this action's signal.

result is found, not_found_in_source or cannot_check. not_found_in_source REQUIRES source { kind, detail } AND period { from, to } — otherwise 400 CHECK_SCOPE_REQUIRED, because a statement without a scope would read as a claim about a third party. cannot_check REQUIRES reason (no_access, source_unknown, needs_human); with no_access a NEW delegation is opened for the same source and period, requiring the missing capability. found closes NOTHING and records NO signal: without signalId the check carries pending_link true and next_move points at POST /actions/{id}/satisfy-signal. Every check records its actor and one domain event, and can never be edited or deleted. The number of checks NEVER escalates anything.