fix(studio): author every new tween against one element

U3 fixed "add keyframe at playhead" widening a write to every sibling
sharing a class, but wired writeTargetSelector into only two paths. The
same bug was still reachable from the add-animation button, drag, resize,
rotate, gesture recording, and the property panel: each derived its target
from selectorFromSelection, which hands back a bare class for an id-less
element, so one edit authored a tween over all five siblings and the
timeline collapsed their rows into one.

Route every path that authors a NEW tween through the existing ladder:

- ensureElementAddressable now accepts selection.selector only when it
  addresses exactly one element, so the id-minting fallback right below it
  (previously unreachable whenever any selector was present) does the work.
- gsapDragCommit's five new-tween branches go through one newTweenTarget
  helper; instant patches reuse the written target so the runtime moves the
  element the source write names.
- useGestureCommit and useAnimatedPropertyCommit keep the existing selector
  for matching/retargeting and author new tweens with a separate write
  selector.

Retargets of an EXISTING tween are deliberately untouched: they keep
anim.targetSelector, so a tween aimed at a whole group stays aimed at it.

Narrowing the write alone regressed idempotency, verified by test: the
"is there already a write for this element" lookups matched targetSelector
by string, so the next nudge missed the write it had just made and appended
a second, conflicting one. The read half now falls back to the live DOM
(tweenTargetsElement, same contract as getAnimationsForElement), which also
still matches a deliberate group tween.

Tests reproduce each site through a real writer, re-parse with the real
parser, and resolve through resolveSelectorElementIds (what feeds the
keyframe cache and the lanes), plus pins for the new-tween vs
retarget-existing distinction so a future change cannot collapse the two.

packages/studio/src/hooks/gsapRuntimeBridge.ts

@@ -213,7 +213,7 @@ export async function tryGsapDragIntercept(
     const existingSet =
       posAnim && isInstantHold(posAnim) && posAnim.targetSelector === selector
         ? posAnim
-        : findExistingPositionWrite(resolvedAnimations, selector);
+        : findExistingPositionWrite(resolvedAnimations, selector, selection.element);
     await commitStaticGsapPosition(selection, offset, gsapPos, selector, existingSet, {
       commitMutation,
       fetchAnimations: fetchFallbackAnimations,
@@ -301,7 +301,8 @@ export async function tryGsapRotationIntercept(
   // rotation set in place, else add a new one. This replaces the old
   // `--hf-studio-rotation` CSS-var fallback (the same dual-channel bug class).
   if (!anim || isInstantHold(anim)) {
-    const existingSet = anim ?? findRotationSetAnimation(resolvedAnimations, selector);
+    const existingSet =
+      anim ?? findRotationSetAnimation(resolvedAnimations, selector, selection.element);
     await commitStaticGsapRotation(selection, newRotation, selector, existingSet, {
       commitMutation,
       fetchAnimations: fetchFallbackAnimations,