Clear references to a template object in every scene on delete (#269)

Deleting an object in a container tab only cleared references inside
the template. Placements in scenes and other containers kept their
instance edit and showed "(missing)" in the inspector.

- shownObjectIds records which objects every holder shows before the
  delete. dropReferencesToGone compares that with the state after it
  and clears each reference that no longer resolves.
- A reference is resolved from its own place with referenceCandidates,
  the same way the runtime does. One rule covers instance edits,
  nested placements and full ids like p~crate.
- A cleared instance edit is dropped when it now says the same as the
  template's own value. Without such a value a marked None stays, as
  if picked by hand, and Reset hands the row back to the template.
- All cleared references come back with a single undo.
- References that were already dead in saved projects stay untouched.

Findings, not fixed here:
- A reference from container D into a nested placement q is stored as
  q~<id>, but the copy is named r~q/<id> in every placement of D. The
  runtime finds nothing, so that reference is dead from the start.
- Delete asset removes placed objects by cascade but does not clear
  references to them.

Tests:
- Unit object-references: 17 passed (13 + 4)
- Gate: 198 files, 2873 tests, exit 0
- e2e 1-17,57: 564 checks passed, 57 has 13 (+1 for 57h)
- Five unit counter-probes and one browser counter-probe fail as
  expected
- e2e total 1607 is computed, not measured in a full run

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

tools/e2e/57-object-references.mjs

@@ -9,6 +9,8 @@
  * a right-click and a second press of the eyedropper leave it as it was. From a list item in a
  * Variables tab it goes to the scene view and comes back with Take.
  *
+ * Removing an object in the Container tab empties a reference a placement of it holds (#269).
+ *
  * Everything it adds carries an `or57` id and goes again at the end.
  */
 import { LANES, MAKE_OBJECT } from './lib/scaffold.mjs';
@@ -268,6 +270,76 @@ export async function objectReferences(t) {
     JSON.stringify({ tabShown, inTab, fromList, listCandidate, backIn, crowd }),
   );
 
+  // 57h. A container placed in the scene, its Aim pointed at its own Crate there (#269). Removing
+  //      the Crate in the Container tab empties that reference too: the row says None, not
+  //      (missing). One undo brings both back.
+  await t.showTab('scene');
+  await mutate(`
+    const p = __gm.state.project;
+    const scene = __gm.state.activeScene;
+    ${MAKE_OBJECT}
+    const sprite = p.assets.find((a) => a.type === 'sprite');
+    p.assets.push({
+      id: 'or57-box',
+      type: 'container',
+      name: 'Box 57',
+      props: {},
+      objects: [make('or57-t-aim', 'or57-aim', '', 0, 'Aim 57'), make('or57-t-crate', sprite.id, '', 50, 'Crate 57')],
+    });
+    const placed = make('or57-placed', 'or57-box', '', ${LANES.containerReference}, 'Box 57');
+    placed.instanceEdits = {
+      'or57-t-aim': { position: {}, overrides: { target: { $object: 'or57-t-crate' } } },
+    };
+    scene.objects.push(placed);
+  `);
+  const placedTarget = () =>
+    json(
+      "__gm.state.activeScene.objects.find((o) => o.id === 'or57-placed')?.instanceEdits['or57-t-aim']?.overrides.target ?? null",
+    );
+  const targetRow = async () => {
+    const field = await json("__gm.inspectorField('target')");
+    const hits = (await json("__gm.locateAll('None')")) ?? [];
+    return {
+      none: hits.some((p) => field && p.x >= field.left && Math.abs(p.y - field.y) < 12),
+      missing: await inInspector('(missing)'),
+      crate: await inInspector('Crate 57'),
+    };
+  };
+  const placedAim = { kind: 'object', holderId: sceneId, objectId: 'or57-placed~or57-t-aim' };
+  await select(placedAim);
+  await sleep(200);
+  const rowBefore = await targetRow();
+
+  await select({ kind: 'object', holderId: 'or57-box', objectId: 'or57-t-crate' });
+  await sleep(200);
+  const tabThen = await json('__gm.tab()');
+  await press('Delete');
+  await sleep(150);
+  const templateLeft = await json(
+    "__gm.state.project.assets.find((a) => a.id === 'or57-box').objects.map((o) => o.id)",
+  );
+  const emptied = await placedTarget();
+  await t.showTab('scene');
+  await select(placedAim);
+  await sleep(200);
+  const rowAfter = await targetRow();
+  await shot('04-container-reference');
+  await undo();
+  await sleep(150);
+  const restored = await placedTarget();
+  check(
+    'removing an object in the Container tab empties the placement pointing at it, undo brings it back',
+    rowBefore.crate &&
+      !rowBefore.missing &&
+      tabThen === 'container' &&
+      JSON.stringify(templateLeft) === JSON.stringify(['or57-t-aim']) &&
+      JSON.stringify(emptied) === JSON.stringify({ $object: '' }) &&
+      rowAfter.none &&
+      !rowAfter.missing &&
+      JSON.stringify(restored) === JSON.stringify({ $object: 'or57-t-crate' }),
+    JSON.stringify({ rowBefore, tabThen, templateLeft, emptied, rowAfter, restored }),
+  );
+
   // Clean up. Back to the scene view first: removing Aim 57 closes 57g's Variables tab, and the
   // editor then shows whatever document earlier steps left open, not the scene view.
   await t.showTab('scene');
@@ -276,6 +348,6 @@ export async function objectReferences(t) {
     const p = __gm.state.project;
     const scene = __gm.state.activeScene;
     scene.objects = scene.objects.filter((o) => !o.id.startsWith('or57-'));
-    p.assets = p.assets.filter((a) => a.id !== 'or57-aim');
+    p.assets = p.assets.filter((a) => a.id !== 'or57-aim' && a.id !== 'or57-box');
   `);
 }

tools/e2e/lib/scaffold.mjs

@@ -28,6 +28,7 @@ export const LANES = {
   vectorArt: -5100,
   pixelArt: -5400,
   eyedropper: -5800,
+  containerReference: -6200,
 };
 
 /**