capsite/toybox: a mount outlives its handler: the vat restarts on demand

A handler is a live reference, held by the VatAdmin DO; workerd evicts
an idle DO within seconds, and with it the handler, so a vat's page went
503 almost at once.

createVat now keeps a vat's params when they hold only data and
ingresses (an ingress by its key: the VatAdmin can make it again), and
which vat was given which ingress. A request for a mount with no handler
restarts that vat with its params; it rebuilds its root, serves again,
and the request is answered. A vat whose params hold another peer's
objects isn't restartable yet.

The test crashes its own toybox (a crash ends every session to it), and
the creator waits for the root's handoff before it disconnects, as
scripts/hello.js now does too.

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

toybox/scripts/hello.js

@@ -12,7 +12,8 @@ const name = process.argv[2] ?? "hello";
 const client = await connectClient("admin.sock", `hello-script-${process.pid}`);
 try {
 	const ingress = await E(client.vatAdmin).makeIngress({ vat: name });
-	const { id } = await E(client.vatAdmin).createVat(await readFile("vats/hello.js", "utf8"), { ingress });
+	const { id, root } = await E(client.vatAdmin).createVat(await readFile("vats/hello.js", "utf8"), { ingress });
+	await root; // its handoff, settled before the client goes
 	console.log(`vat ${id}\nhttp://127.0.0.1:8787/?vat=${name}`);
 } finally {
 	client.close();