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/Entity.ts

@@ -1,3 +1,4 @@
+import type { ENTITY_HANDLE } from '../core/graph/NodeContext';
 import type { AssetType } from '../core/model/AssetTypes';
 import type { PropertyBag } from '../core/model/Properties';
 
@@ -8,6 +9,8 @@ import type { PropertyBag } from '../core/model/Properties';
  * editor panels can show and edit them while playing.
  */
 export interface Entity {
+  /** What tells a graph this object is a live entity and not a struct value. */
+  readonly [ENTITY_HANDLE]: true;
   id: string;
   /** Scene object this entity was instantiated from (for selection highlighting). */
   objectId: string;