Let a value be a bundle of named fields

PropertyValue was number | string | boolean | list. A struct joins it, which
costs two things the typechecker found rather than I did.

An object on a data edge used to mean a live entity, so asEntity would have
taken a struct for one and asString would have read .name off it. EntityHandle
now carries a symbol brand – a symbol so no field name can impersonate it, and
it never serializes, which suits a thing that exists only while the game runs.
It is built in exactly one place.

String(value) would have written a struct as [object Object] at 39 call sites.
They go through textOf(), which is identical for primitives.

normalizeGraph dropped everything but primitives from node params, so the List
node lost its value on every save ([3,1,4] round-tripped to undefined).
Variables were spared: they have a schema, params have none.

src/runtime/World.ts

@@ -1,4 +1,4 @@
-import { isHandleEnabled, type PointerState, worldPositionOf } from '../core/graph';
+import { ENTITY_HANDLE, isHandleEnabled, type PointerState, worldPositionOf } from '../core/graph';
 import { newId } from '../core/ids';
 import type { Asset } from '../core/model/Asset';
 import { isPlaceableType } from '../core/model/AssetTypes';
@@ -118,6 +118,7 @@ export class World {
     let entity = this.findByObjectId(object.id);
     if (!entity) {
       entity = {
+        [ENTITY_HANDLE]: true,
         id: `e-${object.id}`,
         objectId: object.id,
         assetId: asset.id,