fix(studio): give the motion path and the fallbacks a one-element target

The narrowing this branch adds missed the motion-path overlay, and every
caller that could not narrow fell back to the exact bare class the narrowing
exists to replace.

- motionPathSelection.selectorFor now goes through writeTargetSelector. It
  feeds both the geometry read and the "set destination" write, so a class
  sibling measured its home off the FIRST sibling and then authored
  add-motion-path onto all of them. The toolbar toggle hides when no
  one-element form exists rather than arming a press that is dropped.
- The five new-tween writers that fell back to the selection's own selector
  now drop the commit instead. A gesture that does not persist reverts on the
  next reload; a tween silently aimed at five elements does not.
- tweenTargetsElement only follows the DOM to a target that matches exactly
  one element. A target the element merely shares with its siblings is a group
  tween, and these callers mutate what they find, so an individual nudge was
  rewriting the group's own tween and moving all five.

packages/studio/src/hooks/useGestureCommit.ts

@@ -178,7 +178,14 @@ export function useGestureCommit({
         showToast("Cannot save — element has no selector", "error");
         return;
       }
-      const writeSelector = writeTargetSelector(sel) ?? selector;
+      // A recorded gesture becomes a NEW tween, so its target must address one
+      // element; the selection's own selector would record the motion onto
+      // every sibling sharing its class (see writeTargetSelector).
+      const writeSelector = writeTargetSelector(sel);
+      if (!writeSelector) {
+        showToast("Cannot save: element has no unique selector", "error");
+        return;
+      }
       if (liveSession.commitMutation) {
         const recStart = recordingStartTimeRef.current;
         const rawKeyframes = sortedPcts.map((pct) => ({