capsite/toybox: vats with code installed at runtime, on workerd

Each vat is a Durable Object that stores code by hash and runs it as a
facet through the Worker Loader: an empty env, no fetch, storage that
carries over upgrades. Two sockets with no configured address: whoever
starts workerd hands them in (systemd socket activation), so the admin
socket is the authority to install code.

Tests run a real workerd: install, upgrade, isolation between vats, no
fetch, the public port can't install, and code and state survive a crash.
They spawn the package binary rather than the .bin shim, which forks
workerd, so killing the shim orphaned it and the test run never ended.

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

toybox/apps/counter-v1.js

@@ -0,0 +1,8 @@
+import { DurableObject } from "cloudflare:workers";
+export class App extends DurableObject {
+  fetch() {
+    const n = (this.ctx.storage.kv.get("n") ?? 0) + 1;
+    this.ctx.storage.kv.put("n", n);
+    return new Response(`v1 count ${n}\n`);
+  }
+}