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/package.json

@@ -4,12 +4,15 @@
 	"private": true,
 	"type": "module",
 	"scripts": {
-		"start": "mkdir -p state && rm -f admin.sock && systemd-socket-activate -l 127.0.0.1:8787 -l ./admin.sock -- node_modules/workerd/bin/workerd serve --experimental config.capnp -S http=3 -S admin=4 -d state=state -e switchboard=unix:./switchboard.sock",
-		"switchboard": "rm -f switchboard.sock && systemd-socket-activate -l ./switchboard.sock -- node_modules/workerd/bin/workerd serve --experimental switchboard.capnp -S http=3",
+		"start": "mkdir -p state && rm -f admin.sock && systemd-socket-activate -l 127.0.0.1:8787 -l ./admin.sock -- node_modules/workerd/bin/workerd serve --experimental config.capnp -S http=3 -S admin=4 -d state=state -e switchboard=unix:./switchboard-public.sock",
+		"switchboard": "rm -f switchboard-public.sock switchboard-local.sock && systemd-socket-activate -l ./switchboard-public.sock -l ./switchboard-local.sock -- node_modules/workerd/bin/workerd serve --experimental switchboard.capnp -S public=3 -S local=4",
 		"test": "node --test"
 	},
 	"devDependencies": {
 		"workerd": "1.20261006.1",
 		"ws": "8.22.0"
+	},
+	"dependencies": {
+		"capnweb": "0.12.0"
 	}
 }