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>

src/ui/widgets/ListRow.ts

@@ -77,6 +77,13 @@ export class ListRow extends Widget {
     return { x: p.x, y: p.y };
   }
 
+  /** Global centre of the hint; null for rows without one (test hook). */
+  hintGlobal(): { x: number; y: number } | null {
+    if (!this.item.hint) return null;
+    const p = this.hint.getGlobalPosition();
+    return { x: p.x + this.hint.width / 2, y: p.y + this.hint.height / 2 };
+  }
+
   /** Whether a click at this local x lands on the chevron rather than on the row. */
   hitsChevron(localX: number): boolean {
     if (!this.chevronSlot || this.item.expanded === undefined) return false;
@@ -115,13 +122,13 @@ export class ListRow extends Widget {
       mark.position.set(right - ICON_SIZE / 2, this.h / 2);
       right -= ICON_SIZE;
     }
-    const marksW = this.w - Theme.padding - right;
-    this.hint.text = this.item.hint ?? '';
-    let hintW = (this.hint.text ? this.hint.width + Theme.padding : 0) + marksW;
+    // An icon at the end stands in the same column on every row; a hint goes in front of it.
     if (iconAtEnd) {
-      this.icon.position.set(this.w - Theme.padding - hintW - ICON_SIZE / 2, this.h / 2);
-      hintW += ICON_SIZE + Theme.padding;
+      this.icon.position.set(right - ICON_SIZE / 2, this.h / 2);
+      right -= ICON_SIZE + Theme.padding;
     }
+    this.hint.text = this.item.hint ?? '';
+    const hintW = (this.hint.text ? this.hint.width + Theme.padding : 0) + this.w - Theme.padding - right;
     this.labelText.text = truncate(this.item.label, this.labelText.style, this.w - x - hintW - Theme.padding);
     this.labelText.position.set(x, centreY(this.h, this.labelText.height));
     this.hint.position.set(Math.round(right - this.hint.width), centreY(this.h, this.hint.height));

src/ui/widgets/ListView.ts

@@ -19,7 +19,7 @@ export interface ListItem {
   overridden?: boolean;
   /** SVG file drawn in front of the label, 16 units around its origin, `currentColor` muted. */
   icon?: string;
-  /** Draws `icon` at the right edge, where a hint would stand, instead of in front of the label. */
+  /** Draws `icon` at the right edge instead of in front of the label; a hint then stands left of it. */
   iconAtEnd?: boolean;
   /** Small pictures (SVG files) at the right edge, right of the hint – states of the row, like a scene's start mark. */
   marks?: readonly string[];
@@ -231,6 +231,11 @@ export class ListView extends Widget {
     return this.rows.find((r) => r.item.id === id)?.iconGlobal() ?? null;
   }
 
+  /** Global centre of a row's hint; null when the row has none (test hook). */
+  hintGlobal(id: string): { x: number; y: number } | null {
+    return this.rows.find((r) => r.item.id === id)?.hintGlobal() ?? null;
+  }
+
   /** Global centre of a row's chevron; null when the row has none. */
   chevronGlobal(id: string): { x: number; y: number } | null {
     return this.rows.find((r) => r.item.id === id)?.chevronGlobal() ?? null;