Event-aware reduction for Hooks v2 command output.
Return value when it is composed only of allowlisted sequences.
Allowed tokens are OSC 0/1/2/9/99/777 (BEL or ST terminated)
and bare BEL. Any other escape or control content rejects the entire value.
Reduce ordered handler results into an event-specific decision.
Validated output and diagnostics from one command handler.
