event-not-state-edge-for-one-shot-effects

Fire a one-shot celebration effect from the click that caused it, never from the state that click produced — an effect watching a boolean for a false→true edge cannot tell a tap from data arriving a beat later, so moving to an already-marked record fires the same edge. Covers the @Stable holder that owns live effects, judging the meaning at tap time, why the previous-value variable is a second source of truth, and…