fix(studio): span the clip on the remaining duration-less tween paths

Two sibling call paths still answered from GSAP's 0.5s default while the
toggle path had moved to the clip-wide fallback.

isPlayheadWithinTween now takes the selection, so the toolbar stops
promising "extends animation" on a duration-less tween whose click actually
toggles an interior keyframe. The arc drag commit resolves its replacement
duration the same way its click-path sibling does, instead of authoring
duration 0.5 and collapsing the arc window.

The flat text section drops its two marker-clearing effects: the autofocus
marker is a ref now, read and cleared by the render that consumes it.

packages/studio/src/player/components/timelineKeyframeIdentity.ts

@@ -5,6 +5,13 @@ export interface TimelineKeyframeTarget {
   animationId?: string;
 }
 
+/**
+ * Note the asymmetry: `tweenPercentage` is optional here and defaults to
+ * `percentage`, but the reader treats both slots as authoritative. Callers must
+ * therefore keep the pair consistent — writing a new `tweenPercentage` without
+ * the matching `percentage` hashes two logically-equal selections to different
+ * keys.
+ */
 export function timelineKeyframeSelectionKey(
   elementId: string,
   target: TimelineKeyframeTarget,