Fix references into nested placements and on asset delete (#285, #286)

Two findings from #269. A reference from a container into a placement
nested in it was dead from the start, and deleting an asset left
references to the objects it took along showing "(missing)".

- A reference is stored the way its holder shows the object. In a
  container tab that is q~x for content of a nested placement q.
- referenceCandidates translates it when the container is placed: in
  placement r, q~x becomes r~q/x. pathOfId and idOfPath in instance.ts
  do the mapping, objectChoices offers the same spelling the other way.
  The rule holds at any depth.
- Storing q/x as proposed in #285 was dropped. The container tab would
  then have had to translate its own ids.
- No migration. All projects and snapshots hold three references, none
  of them with ~ or /.
- Create container lets a reference to content of a member placement
  follow it. It was left dead before.
- Paste lets such a reference point into the copy. It kept pointing at
  the original before. Both go through the new renamedReference.
- Delete asset records what every holder shows before the cascade and
  clears every reference to an object that is gone afterwards, with
  dropReferencesToGone from #269. The row shows None, undo restores
  the references (#286).

Tests:
- Gate: 198 files, 2882 tests (2873 + 9), exit 0
- Unit object-references: 26 passed (17 + 9)
- e2e 1-17,57: 566 checks passed, 57 has +57i and +57j
- Six unit counter-probes fail between 1 and 3 tests each
- The browser counter-probe fails 57i and 57j as described in the
  issues
- e2e total 1609 is computed, not measured in a full run

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

src/core/commands/assetCommands.ts

@@ -15,6 +15,7 @@ import { isFontFile } from '../model/font';
 import { DEFAULT_PALETTE, PIXEL_CANVAS_DEFAULT } from '../model/pixelart';
 import { isSoundFile } from '../model/sound';
 import { removeSubtrees } from '../model/hierarchy';
+import { dropReferencesToGone, shownObjectIds } from '../model/objectRefs';
 import type { PropertyBag, PropertyValue } from '../model/Properties';
 import { objectHolders, type Project } from '../model/Project';
 import {
@@ -30,6 +31,7 @@ import { isTextureFile } from '../model/texture';
 import { cellsWithout } from '../model/tilemap';
 import { coerceValue, defaultsFor, initialOf, initialPropsFor } from '../properties/PropertySchema';
 import { type Removed, removeById, removeFirst, restoreAt } from '../util/list';
+import { Recorder } from '../util/recorder';
 import { type Command, CommandError } from './Command';
 import { requireAsset, requirePropDef, requireStateMachine, requireVariablesAsset } from './require';
 import { SetBagValueCommand } from './SetBagValueCommand';
@@ -212,13 +214,15 @@ export class CreateSoundCommand extends ImportFileCommand<SoundAsset> {
 /**
  * Deletes an asset and, as a cascade, every scene object referencing it, together with the objects
  * parented below those – in container templates as well as in scenes. A tile takes the cells it is
- * painted on along (#268), which would otherwise stay in the file and draw nothing.
+ * painted on along (#268), which would otherwise stay in the file and draw nothing. A reference to
+ * an object that goes is emptied, as when the object is removed by itself (#286).
  */
 export class DeleteAssetCommand implements Command {
   readonly label = 'Delete asset';
   private removed?: Removed<Asset>;
   private removedObjects: { holderId: string; removed: Removed<SceneObject>[] }[] = [];
   private emptied: { layer: TilemapLayer; cells: Record<string, string> }[] = [];
+  private readonly references = new Recorder();
   private previousActiveScene = '';
 
   constructor(private readonly assetId: string) {}
@@ -227,6 +231,7 @@ export class DeleteAssetCommand implements Command {
     const asset = requireAsset(project, this.assetId);
     this.removedObjects = [];
     this.emptied = [];
+    const before = shownObjectIds(project);
     if (asset.type === 'tile')
       for (const map of project.assets) {
         if (map.type !== 'tilemap') continue;
@@ -245,6 +250,7 @@ export class DeleteAssetCommand implements Command {
       this.removedObjects.push({ holderId: holder.id, removed: removeSubtrees(holder, roots) });
     }
     this.removed = removeFirst(project.assets, (a) => a.id === this.assetId);
+    dropReferencesToGone(project, before, this.references);
     this.previousActiveScene = project.activeSceneId;
     if (project.activeSceneId === this.assetId) {
       const next = project.assets.find((a) => a.type === 'scene');
@@ -254,6 +260,7 @@ export class DeleteAssetCommand implements Command {
 
   undo(project: Project): void {
     if (!this.removed) return;
+    this.references.revert();
     restoreAt(project.assets, [this.removed]);
     const holders = new Map(objectHolders(project).map((h) => [h.id, h]));
     for (const entry of this.removedObjects) {

src/core/commands/containerCommands.ts

@@ -1,8 +1,14 @@
 import { newId } from '../ids';
 import type { ContainerAsset } from '../model/Asset';
 import { findObject, removeSubtrees, subtreeIds, subtreeRoots, treeOrder } from '../model/hierarchy';
-import { instanceChildId, type InstanceEdit, isLiveInstance, materializeInstance } from '../model/instance';
-import { anchorReferences, swapReferences } from '../model/objectRefs';
+import {
+  instanceChildId,
+  type InstanceEdit,
+  isLiveInstance,
+  materializeInstance,
+  pathOfId,
+} from '../model/instance';
+import { anchorReferences, renamedReference, swapReferences } from '../model/objectRefs';
 import { apply, invert, multiply, translation } from '../model/matrix';
 import type { Position } from '../model/Properties';
 import { findAsset, findObjectHolder, type Project } from '../model/Project';
@@ -114,12 +120,13 @@ export class CreateContainerCommand implements Command {
     if (!containerParentId && groupId) container.groupId = groupId;
     this.removed = removeSubtrees(holder, [...roots]);
     // A reference to a member follows it (#258): inside the template to its template id, from the
-    // rest of the holder to what the placed container shows of it.
-    const templateIdOf = (ref: string) => templateIds.get(ref);
+    // rest of the holder to what the placed container shows of it – content of a member placement
+    // included (#285).
+    const templateIdOf = (ref: string) => renamedReference(ref, templateIds);
     swapReferences(asset.objects, templateIdOf, this.references);
     const shown = (ref: string) => {
       const moved = templateIdOf(ref);
-      return moved && instanceChildId(this.objectId, moved);
+      return moved && instanceChildId(this.objectId, pathOfId(moved));
     };
     swapReferences(holder.objects, shown, this.references);
     project.assets.push(asset);

src/core/commands/pasteCommands.ts

@@ -5,7 +5,7 @@ import { newId } from '../ids';
 import type { GraphClip, ObjectClip } from '../model/clipboard';
 import { findGroup } from '../model/hierarchy';
 import { isLiveInstance } from '../model/instance';
-import { swapReferences } from '../model/objectRefs';
+import { renamedReference, swapReferences } from '../model/objectRefs';
 import { Recorder } from '../util/recorder';
 import { placementRejection } from '../model/placement';
 import type { Vec2 } from '../model/Properties';
@@ -79,7 +79,7 @@ export class PasteObjectsCommand implements Command {
       return copy;
     });
     // A reference among the copied objects points at the copy; one out of them stays (#258).
-    swapReferences(pasted, (ref) => this.ids.get(ref), new Recorder());
+    swapReferences(pasted, (ref) => renamedReference(ref, this.ids), new Recorder());
     holder.objects.push(...pasted);
   }