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/components/StudioRightPanel.tsx

@@ -93,6 +93,7 @@ export function StudioRightPanel({
     domEditGroupSelections,
     copiedAgentPrompt,
     clearDomSelection,
+    handleUngroupSelection,
     handleDomStyleCommit,
     handleDomAttributeCommit,
     handleDomAttributeLiveCommit,
@@ -241,6 +242,7 @@ export function StudioRightPanel({
       multiSelectCount={domEditGroupSelections.length}
       copiedAgentPrompt={copiedAgentPrompt}
       onClearSelection={clearDomSelection}
+      onUngroup={handleUngroupSelection}
       onSetStyle={handleDomStyleCommit}
       onSetAttribute={handleDomAttributeCommit}
       onSetAttributeLive={handleDomAttributeLiveCommit}