Drag a whole selection and join existing groups in the hierarchy (#241)

Three follow-ups to the hierarchy groups. Dragging a row that belonged
to a multi-selection moved only that one row. The group dialog could
only create a new group. The group row drew its folder in front of the
label, unlike every other row in the hierarchy.

- Dragging a selected row moves every selected object. This holds for
  reordering, dropping into a group or between its members, dragging
  out of a group and dropping onto an object as new parent.
- The moved objects land side by side in hierarchy order, whichever
  row was grabbed. The whole move is one undo step.
- A selected child of a selected object travels with its parent, not
  on its own. Dropping the selection onto its own rows does nothing.
- Dragging a row that is not selected still moves only that row.
- hierarchyDrop.moveObjects replaces the single-object rule. One object
  is the case with one element, so the old drop tests hold unchanged.
- The group dialog has a "Group" dropdown: New group or an existing
  group. Picking a group greys out the name and turns the button into
  "Add". A group that holds all chosen objects already is not offered.
- Rename keeps using the dialog without the dropdown.
- ListRow draws an end icon at the very right and the hint left of it.
  Only the hierarchy uses iconAtEnd, so other lists do not change.
- New test hook __gm.hierarchyHint() gives the centre of a row's hint.

Not clicked in the browser: dragging a selection in the container tab
and onto an object as parent. Only the unit tests cover these.

Tests:
- ./check.sh: 178 files, 2564 tests, CHECK EXIT 0
- e2e 1 + 52: ALL PASSED, 27 checks
- 7 new unit tests in hierarchy-drop.test.ts, 4 new e2e checks in
  52i and 52j with real clicks and a real drag
- 4 counter-probes bite: drop rule ignores the selection (5 unit tests
  fail), icon column not kept free, dialog ignores the picked group,
  panel ignores the selection (only o2 lands in the group)

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

tools/e2e/52-hierarchy-groups.mjs

@@ -169,4 +169,76 @@ export async function hierarchyGroups(t) {
     JSON.stringify({ down, primaryAfter, up, rows: await rowIds() }),
   );
   await evaluate('__gm.state.select(null); __gm.hierarchyCollapseAll()');
+
+  // 52i. A group "Pile" of o5 alone; its folder stands in the icon column of the objects, the
+  // count left of it. Then o1 and o2 join it through the dialog's group dropdown.
+  await select({ kind: 'object', holderId: 'scene1', objectId: 'o5', objectIds: ['o5'] });
+  const lone = await rowAt('o5');
+  await click(lone.x, lone.y, 'right');
+  await clickText('Group…');
+  await typeText('Pile');
+  await press('Enter');
+  const pile = (await scene()).groups?.[0];
+  const pileRow = `group:${pile?.id}`;
+  const folder = await json(`__gm.hierarchyIcon(${JSON.stringify(pileRow)})`);
+  const count = await json(`__gm.hierarchyHint(${JSON.stringify(pileRow)})`);
+  const objectIcon = await json('__gm.hierarchyIcon("o1")');
+  check(
+    "a group's folder stands in the objects' icon column, its count left of it",
+    folder !== null &&
+      objectIcon !== null &&
+      count !== null &&
+      Math.abs(folder.x - objectIcon.x) < 1 &&
+      count.x < folder.x - 8,
+    JSON.stringify({ folder, objectIcon, count }),
+  );
+  await select({ kind: 'object', holderId: 'scene1', objectId: 'o1', objectIds: ['o1', 'o2'] });
+  const joiner = await rowAt('o1');
+  await click(joiner.x, joiner.y, 'right');
+  await clickText('Group 2 objects…');
+  await clickText('New group');
+  const choices = (await json('__gm.menuRows()')) ?? [];
+  // The last "Pile" drawn is the menu's, above the hierarchy row of the same name.
+  const pick = await json('__gm.locate("Pile", true)');
+  await click(pick.x, pick.y);
+  await shot('52-join-group');
+  await clickText('Add');
+  const joined = await scene();
+  check(
+    'the Group dialog puts the objects into an existing group picked from its dropdown',
+    same(choices, ['New group', 'Pile']) &&
+      joined.groups?.length === 1 &&
+      same(
+        joined.objects.filter((o) => o.groupId === pile?.id).map((o) => o.id),
+        ['o1', 'o2', 'o5'],
+      ),
+    JSON.stringify({ choices, groups: joined.groups, objects: joined.objects.map((o) => [o.id, o.groupId]) }),
+  );
+  await undo();
+
+  // 52j. Dragging one row of a multi-selection takes the others along – here onto the group row.
+  await select({ kind: 'object', holderId: 'scene1', objectId: 'o1', objectIds: ['o1', 'o2'] });
+  const grab = await rowAt('o2');
+  const into = await rowAt(pileRow);
+  await drag(grab.x, grab.y, into.x, into.y);
+  const dragged = await scene();
+  const inPile = dragged.objects.filter((o) => o.groupId === pile?.id).map((o) => o.id);
+  await undo();
+  const undone = await scene();
+  check(
+    'dragging a selected row moves the whole selection, in one undo step',
+    same(inPile, ['o5', 'o1', 'o2']) &&
+      same(
+        undone.objects.filter((o) => o.groupId === pile?.id).map((o) => o.id),
+        ['o5'],
+      ),
+    JSON.stringify({ inPile, undone: undone.objects.map((o) => [o.id, o.groupId]) }),
+  );
+  await undo();
+  check(
+    'undo takes the group of o5 back too',
+    (await scene()).groups === undefined,
+    JSON.stringify((await scene()).groups),
+  );
+  await evaluate('__gm.state.select(null); __gm.hierarchyCollapseAll()');
 }