feat(studio): element groups — studio UI (#1759)

* feat(studio): element groups — source mutations

Wrap/unwrap source mutations (group geometry, the wrap-elements / unwrap-elements
routes) that the studio group feature is built on. Studio UI lands in the next PR.

* fix(studio): hoverable group interior + non-sticky drill-in

Two group selection bugs with animated members:

1) Empty space inside a group's overlay didn't hover/select the group. Members
   animated outside the wrapper's static box (110px box vs 340px member union),
   so elementsFromPoint hit only the full-bleed background there. Add a
   member-union hit-test fallback: a point inside a group's live member bounds
   resolves to that group (innermost wins).

2) After drilling into a group and selecting a child, nothing else was
   selectable — out-of-scope resolved to null. Make drill-in non-sticky:
   interacting outside the drilled group re-resolves normally and exits the
   drill-in, so a later click on the group selects it as a unit again.

packages/studio/src/App.tsx

@@ -258,6 +258,8 @@ export function StudioApp() {
     onResetKeyframes: () => resetKeyframesRef.current(),
     onDeleteSelectedKeyframes: () => deleteSelectedKeyframesRef.current(),
     onAfterUndoRedo: () => invalidateGsapCacheRef.current(),
+    onGroupSelection: () => domEditSessionRef.current.handleGroupSelection(),
+    onUngroupSelection: () => domEditSessionRef.current.handleUngroupSelection(),
     activeCompPath,
     forceReloadSdkSession: sdkHandle.forceReload,
     onToggleRecording: STUDIO_KEYFRAMES_ENABLED