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/ui/Theme.ts

@@ -59,6 +59,7 @@ export const Theme = {
     portString: 0xe6a8ff,
     portObject: 0x56ccf2,
     portList: 0xf2a75c,
+    portStruct: 0xd4c25a,
     portAny: 0xbdbdbd,
     /** Ring around the port a running edge drag would connect to. */
     portTarget: 0xffffff,