rule-21 standalone-ack gate misses 'ack <noun>' shape

kpi-coach observed pm-mars-cc's bare 'ack drain' (kind=status, 13x/24h) slipping both rule-21's standalone-ack gate (hub.mjs ~5462, ACK_STANDALONE_RE requires the WHOLE body be one of the listed tokens - 'ack drain' has a trailing word so the anchor $ fails) and PHATIC_STRICT_RE (different phrase list entirely). Fix: extend ACK_STANDALONE_RE or add a new class for 'ack <noun phrase>' bodies with no further content. Low priority, not urgent, low volume.