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/core/graph/nodes/variable/GetVariable.ts

@@ -1,6 +1,7 @@
 import type { NodeDef } from '../../NodeRegistry';
 import { any, variableParam } from '../shared';
 
+import { textOf } from '../../../model/Properties';
 export const GetVariable: NodeDef = {
   type: 'GetVariable',
   label: 'Get variable',
@@ -10,6 +11,6 @@ export const GetVariable: NodeDef = {
   outputs: [any('value')],
   params: [variableParam],
   run: (ctx, _inputs, params) => ({
-    outputs: { value: ctx.variables[String(params.variable ?? '')] ?? null },
+    outputs: { value: ctx.variables[textOf(params.variable ?? '')] ?? null },
   }),
 };

src/core/graph/nodes/variable/SetVariable.ts

@@ -2,6 +2,7 @@ import type { NodeDef } from '../../NodeRegistry';
 import { writeTyped } from '../../values';
 import { any, exec, variableParam } from '../shared';
 
+import { textOf } from '../../../model/Properties';
 export const SetVariable: NodeDef = {
   type: 'SetVariable',
   label: 'Set variable',
@@ -11,7 +12,7 @@ export const SetVariable: NodeDef = {
   outputs: [exec()],
   params: [variableParam],
   run: (ctx, inputs, params) => {
-    const key = String(params.variable ?? '');
+    const key = textOf(params.variable ?? '');
     if (key) writeTyped(ctx.variables, key, inputs.value);
     return { next: 'exec' };
   },