Group scene objects in the hierarchy, for the editor only (#241)

A scene with a hundred crates fills the hierarchy with a hundred rows.
Groups bundle top-level objects under one folder row that folds away.
They exist for the editor only. Parenting, positions and the game do
not see them.

- Model: a scene or container template holds a list of groups, an
  object may carry a groupId. Old projects load unchanged. Loading
  drops empty groups and members that are no longer top level.
- Only top-level objects can join a group. Parenting a member takes it
  out of the group, and Undo puts it back. There are no groups in
  groups.
- Order: treeOrder lists the members together, where the first one
  stands. The same order decides drawing at equal z, in the editor and
  in the game, so the hierarchy and the picture cannot disagree. The
  stored object list stays untouched when grouping.
- Hierarchy: a group row shows a folder icon, the name and the member
  count. Clicking it selects all members and keeps it folded. Selecting
  a single member elsewhere unfolds it. Folding lasts for the session.
- Create with "Group..." in the object menu. A name dialog asks first,
  so creating is one Undo step and Cancel creates nothing.
- The group menu has Select members, Rename... and Ungroup. The object
  menu adds "Remove from group".
- Drag and drop: onto the group row appends, into a gap between members
  inserts there, next to a row outside leaves the group. Dragging the
  group row moves all members as a block. The drop rules live in
  hierarchyDrop.ts as a pure function.
- Copy and paste keeps a top-level copy in the group of its original.
  A container built from group members takes their place in the group.
- Clicking an object that is already selected together with others now
  keeps only that object selected. ListView narrows the selection on
  release, so a drag starting on such a row keeps the selection.

Not checked in the browser: groups in the container tab, "Remove from
group", Ctrl-click on a group row and a drag out of a multi-selection.
The code path covers them.

Tests:
- check.sh: 177 files, 2552 tests, CHECK EXIT 0
- e2e 1, 51, 52: ALL PASSED, 31 checks (new module 52)
- 19 new unit tests in object-groups.test.ts and hierarchy-drop.test.ts
- 9 counter-probes, all fail as expected (7 unit, 2 in the browser)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -43,7 +43,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1432 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1442 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs