Let a container sort its content with its parent (#243)

A container is not drawn, so it had no place in the draw order. The
chain from its content to the parent of the container broke there, and
a container under a character could not be drawn together with it.

- A container gets the switch Sort with parent under Z. It defaults to
  off, also for new containers, so projects draw as they did.
- With the switch on, the whole content is drawn in the slot of the
  container's parent. The content's own switch does not count there.
- Containers in a row pass the content on as long as each has the
  switch on. On the top level or under a tilemap it has no effect.
- drawSlots follows the new sortsWith field of a draw item. Scene view,
  game and picking all use drawSlots, so all three agree.

Found on the way: a scene object hung directly under a container
spells the container out, and its template content is dropped. That is
the existing spelledOut rule, the tests now keep both cases apart.

Tests:
- ./check.sh: 178 files, 2573 tests, CHECK EXIT 0
- 7 new unit tests in sort-with-parent.test.ts: the switch, template
  content, override per container, a chain, top level and tilemap,
  game and picking
- e2e 5b-iii: a real click on the switch in the inspector, with undo
- 4 counter-probes over #242 and #243 bite (2 unit, 2 browser)

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

src/core/properties/PropertySchema.ts

@@ -274,13 +274,26 @@ export const SORT_PROPS: PropertySchema = [
   { key: SORT_WITH_PARENT_KEY, label: 'Sort with parent', kind: 'boolean', default: false, initial: true },
 ];
 
+/**
+ * A container is not drawn, but with the switch on it passes its content on to its own parent
+ * (#243). It starts off, so content placed before draws as it did.
+ */
+const CONTAINER_SORT_PROPS: PropertySchema = [
+  { key: SORT_WITH_PARENT_KEY, label: 'Sort with parent', kind: 'boolean', default: false },
+];
+
 /** A tilemap has no single place in the draw order, and the other types are not drawn at all. */
-const SORT_TYPES: ReadonlySet<AssetType> = new Set(['sprite', 'text', 'particles']);
+const SORT_TYPES: ReadonlySet<AssetType> = new Set(['sprite', 'text', 'particles', 'container']);
 
 export function hasSortProps(type: AssetType): boolean {
   return SORT_TYPES.has(type);
 }
 
+function sortPropsOf(type: AssetType): PropertySchema {
+  if (!hasSortProps(type)) return [];
+  return type === 'container' ? CONTAINER_SORT_PROPS : SORT_PROPS;
+}
+
 export function isSortProp(key: string): boolean {
   return key === SORT_WITH_PARENT_KEY;
 }
@@ -640,7 +653,7 @@ const FULL_SCHEMAS: Record<AssetType, PropertySchema> = Object.fromEntries(
       ? [
           ...COMMON_PROPS,
           ...(hasTransformProps(type) ? TRANSFORM_PROPS : []),
-          ...(hasSortProps(type) ? SORT_PROPS : []),
+          ...sortPropsOf(type),
           ...ASSET_SCHEMAS[type],
         ]
       : ASSET_SCHEMAS[type],