Resolve container content from the template instead of copying it

A scene now stores only the container object; its content is resolved from
the template on every read, with deviations kept as sparse instanceEdits on
the container object. This drops the propagation bookkeeping entirely --
templateSync.ts is gone and the three structural gaps (add, remove, reparent)
cannot diverge any more.

Values stay editable on an instance, structure does not; Unpack writes the
content into the scene, keeping ids. Play mode bakes containers into the
throwaway copy before entities are built. No migration: formatVersion is 2
and the seeders write the new format.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/commands/variableCommands.ts

@@ -1,5 +1,6 @@
 import type { PropertyValue } from '../model/Properties';
-import { findUsages, type Project } from '../model/Project';
+import { overrideSites } from '../model/instance';
+import type { Project } from '../model/Project';
 import { isCommonProp, type PropertyDef, type VariableKind, variableDef } from '../properties/PropertySchema';
 import { type Removed, removeFirst, restoreAt } from '../util/list';
 import { type Command, CommandError } from './Command';
@@ -39,7 +40,7 @@ export class AddVariableCommand implements Command {
 export class RemoveVariableCommand implements Command {
   readonly label: string;
   private removed?: { def: Removed<PropertyDef>; value: PropertyValue | undefined };
-  private overrides: { holderId: string; objectId: string; value: PropertyValue }[] = [];
+  private overrides: { holderId: string; objectId: string; path: string; value: PropertyValue }[] = [];
 
   constructor(
     private readonly assetId: string,
@@ -55,11 +56,11 @@ export class RemoveVariableCommand implements Command {
     this.removed = { def, value: asset.props[this.key] };
     delete asset.props[this.key];
     this.overrides = [];
-    for (const { holder, object } of findUsages(project, this.assetId)) {
-      const value = object.overrides[this.key];
+    for (const site of overrideSites(project, (id) => id === this.assetId)) {
+      const value = site.bag[this.key];
       if (value === undefined) continue;
-      this.overrides.push({ holderId: holder.id, objectId: object.id, value });
-      delete object.overrides[this.key];
+      this.overrides.push({ holderId: site.holder.id, objectId: site.object.id, path: site.path, value });
+      delete site.bag[this.key];
     }
   }
 
@@ -68,10 +69,12 @@ export class RemoveVariableCommand implements Command {
     const asset = requireBehavior(project, this.assetId);
     restoreAt(asset.variables, [this.removed.def]);
     if (this.removed.value !== undefined) asset.props[this.key] = this.removed.value;
-    const usages = findUsages(project, this.assetId);
+    const sites = overrideSites(project, (id) => id === this.assetId);
     for (const o of this.overrides) {
-      const object = usages.find((u) => u.holder.id === o.holderId && u.object.id === o.objectId)?.object;
-      if (object) object.overrides[this.key] = o.value;
+      const site = sites.find(
+        (s) => s.holder.id === o.holderId && s.object.id === o.objectId && s.path === o.path,
+      );
+      if (site) site.bag[this.key] = o.value;
     }
   }
 }
@@ -109,10 +112,10 @@ export class RenameVariableCommand implements Command {
       asset.props[to] = asset.props[from]!;
       delete asset.props[from];
     }
-    for (const { object } of findUsages(project, this.assetId)) {
-      if (object.overrides[from] === undefined) continue;
-      object.overrides[to] = object.overrides[from]!;
-      delete object.overrides[from];
+    for (const { bag } of overrideSites(project, (id) => id === this.assetId)) {
+      if (bag[from] === undefined) continue;
+      bag[to] = bag[from]!;
+      delete bag[from];
     }
     // Every node that picks a variable stores it under the param key `variable`.
     for (const node of asset.graph.nodes) if (node.params.variable === from) node.params.variable = to;