Add ActionResult::yield() and ExecutionState::YIELD

[benrowe]

No description provided.

[benrowe]

Re the @phpstan-impure comment on ActionExecutionService::executeAction(): removed in the latest commit. The annotation was working around a PHPStan false positive where it cached isYielded() as always-false after a call it treated as pure. Replaced structurally — executeNextAction() now captures the ExecutionState return value from executeAction() and checks it via a new ExecutionState::isYield() helper (consistent with GateResult::shouldSkipAction() / shouldStopTransition()), so the context is never re-queried after the call.

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