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/shared.ts

@@ -9,6 +9,7 @@ export const bool = (key: string, label = key): PortDef => ({ key, label, type:
 export const str = (key: string, label = key): PortDef => ({ key, label, type: 'string' });
 export const obj = (key: string, label = key): PortDef => ({ key, label, type: 'object' });
 export const list = (key: string, label = key): PortDef => ({ key, label, type: 'list' });
+export const struct = (key: string, label = key): PortDef => ({ key, label, type: 'struct' });
 export const any = (key: string, label = key): PortDef => ({ key, label, type: 'any' });
 
 export const withDefault = (port: PortDef, value: PropertyValue): PortDef => ({ ...port, default: value });