capsite/toybox: egress through Flower-style switchboard handles

The switchboard now serves a Cap'n Web Handle on each of its sockets:
public (no loopback or private addresses) and local. A socket's labels
are fixed in config, so holding the socket's fd is holding that handle;
constrain(labels) returns a narrower one, as Flower's Constrain returns
a narrower fd. Labels only accumulate: a missing key matches anything,
and a key both sides set must agree (labels.js, shared by both).

The switchboard enforces the labels; toybox's Egress no longer checks
URLs itself. It opens a session to its one switchboard socket, constrains
the handle to the vat's grant, and forwards through it; a WebSocket
crosses the session as a pair of message streams. A vat narrows its own
grant with env.egress.constrain().

Grants are now labels as JSON. One stored before, a bare host[:port],
reads as {host, port}.

pnpm start and the systemd units give toybox the public handle; the
switchboard unit binds the local one in its own 0700 runtime directory.
The tests give their toybox the local handle to reach an echo server,
and check that a toybox holding only the public one never reaches it.

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

toybox/apps/egress-probe.js

@@ -1,13 +1,15 @@
 import { DurableObject } from "cloudflare:workers";
 
-/** `?url=<target>` fetches it through `env.egress`; with `&ws=1`, opens a WebSocket there and reports the echo of "hello". */
+/** `?url=<target>` fetches it through `env.egress` (narrowed by `&constrain=<labels as JSON>`, if given); 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");
-		const { egress } = this.env;
-		if (!egress) return new Response("no egress granted\n");
+		if (!this.env.egress) return new Response("no egress granted\n");
 		try {
+			const egress = params.has("constrain")
+				? await this.env.egress.constrain(JSON.parse(params.get("constrain")))
+				: this.env.egress;
 			if (!params.has("ws")) return new Response(`got: ${await (await egress.fetch(target)).text()}\n`);
 			const resp = await egress.fetch(target, { headers: { Upgrade: "websocket" } });
 			const ws = resp.webSocket;