capsite/toybox: OCapN vats; createVat takes remote objects

The admin socket now also speaks OCapN over Cap'n Web: a WebSocket there
reaches the VatAdmin DO, an OCapN peer whose bootstrap (swissnum
vatAdmin) offers createVat(src, params). params may hold remote objects;
the vat's root comes back as one.

Each OCapN vat is its own peer: SES and @endo/ocapn run in its facet
(vat-runtime.js, bundled with esbuild), around the vat's own module,
which exports buildRootObject(vatPowers, params). Passing a client's
object to a vat, or a vat's root to the client, is an OCapN handoff, so
the receiver dials the exporter. Nothing here can dial out, so the
VatAdmin is also the router: a client registers its gateway under a
designator, a vat's designator is its id, and every peer reaches every
other through it. The netlayer is ertp-clerk's, made to work over Workers
RPC stubs and plain promises as well as Cap'n Web.

Not yet: sturdyrefs that survive a restart, an adminNode, and grants
(egress, mount) for OCapN vats. App vats (export class App) still work
beside them.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

toybox/vats/greeter.js

@@ -0,0 +1,6 @@
+/** A vat that greets through the `carol` it was created with. */
+export const buildRootObject = ({ E, Far }, { carol }) =>
+	Far("greeterRoot", {
+		greetVia: (name) => E(carol).greet(name),
+		hello: () => "hello from the vat",
+	});