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/systems/BehaviorSystem.ts

@@ -9,7 +9,7 @@ import {
 } from '../../core/graph';
 import { stepKeyRepeat } from '../../core/graph/keyRepeat';
 import { behaviorLookup } from '../../core/model/behaviorPrograms';
-import type { PropertyBag } from '../../core/model/Properties';
+import { type PropertyBag, textOf } from '../../core/model/Properties';
 import { findAsset, type Project } from '../../core/model/Project';
 import type { Entity } from '../Entity';
 import type { World } from '../World';
@@ -95,7 +95,7 @@ export class BehaviorSystem implements System {
 
   update(world: World, dt: number): void {
     this.syncInstances(world);
-    const keyOf = (node: { params: PropertyBag }) => String(node.params.key ?? '');
+    const keyOf = (node: { params: PropertyBag }) => textOf(node.params.key ?? '');
     for (const instance of this.instances) {
       if (!world.entities.includes(instance.entity)) continue; // destroyed during play
       if (!isHandleEnabled(instance.entity)) continue; // switched off (itself or a parent)
@@ -136,7 +136,7 @@ export class BehaviorSystem implements System {
    * in `nodeState`, so two nodes on the same key repeat on their own schedules.
    */
   private fireKeyDown(i: GraphInterpreter, ctx: NodeContext, node: GraphNode, dt: number): void {
-    const key = String(node.params.key ?? '');
+    const key = textOf(node.params.key ?? '');
     const stored = i.nodeState.get(node.id);
     const { state, fires } = stepKeyRepeat(
       stored ? { held: stored.held ?? 0, repeats: stored.repeats ?? 0 } : null,