Take the cross-cutting queries out of the model's base module

findPropertyUsages walks override sites, so Project.ts imported instance.ts
while instance.ts imported Project.ts – the repo's only value-level import
cycle. The queries now sit in usages.ts and Project.ts is a base type module
again. requireScene has had no caller since holderId replaced sceneId.

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

src/core/model/instance.ts

@@ -256,8 +256,6 @@ export function instanceTargetOf(
   };
 }
 
-// --- writing -----------------------------------------------------------------------------------
-
 /**
  * Read/write access to a placed object, whether it is stored in the holder or resolved from a
  * container template. `object` is the current state either way; the setters keep an instance edit
@@ -421,8 +419,6 @@ export function overrideSites(project: Project, wanted: (assetId: string) => boo
   return sites;
 }
 
-// --- unpacking ---------------------------------------------------------------------------------
-
 /**
  * The resolved content of an instance as objects that can be stored. The derived ids are kept:
  * they are unique already, and anything pointing at one – the selection, a running game, a check