capsite/toybox: a switchboard, the vats' only way out

A vat granted a host (PUT /<vat>?egress=<host> on the admin socket) gets
Egress as its globalOutbound, with the host in ctx.props where the vat
can't change it. Egress forwards matching requests, WebSocket upgrades
included, to the switchboard and refuses the rest with a 403; with no
grant, fetch still throws.

The switchboard is a separate workerd: a Switchboard DO that calls fetch
and returns the response, so a 101's webSocket passes through. toybox
reaches it over a unix socket in HTTP proxy style, which keeps the full
URL, scheme included. Its network service is the outer fence, with TLS
trusting the browser CAs; systemd units for it join toybox's, and only
the toybox group can open its socket.

The loaded Worker is now named by vat and incarnation, not by code hash:
the same code with a different grant must not share a Worker. The new
incarnation table sits beside the old ones; vats installed before it
read as incarnation 0 with no grant.

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

toybox/apps/egress-probe.js

@@ -0,0 +1,23 @@
+import { DurableObject } from "cloudflare:workers";
+
+/** `?url=<target>` fetches it; with `&ws=1`, opens a WebSocket there and reports the echo of "hello". */
+export class App extends DurableObject {
+	async fetch(request) {
+		const params = new URL(request.url).searchParams;
+		const target = params.get("url");
+		try {
+			if (!params.has("ws")) return new Response(`got: ${await (await fetch(target)).text()}\n`);
+			const resp = await fetch(target, { headers: { Upgrade: "websocket" } });
+			const ws = resp.webSocket;
+			if (!ws) return new Response(`no websocket: ${resp.status} ${await resp.text()}`);
+			ws.accept();
+			const reply = new Promise((resolve) => ws.addEventListener("message", (e) => resolve(e.data)));
+			ws.send("hello");
+			const got = await reply;
+			ws.close();
+			return new Response(`echo: ${got}\n`);
+		} catch (e) {
+			return new Response(`refused: ${e.message}\n`);
+		}
+	}
+}