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/config.capnp

@@ -10,8 +10,8 @@ const config :Workerd.Config = (
   services = [
     (name = "main", worker = .supervisor),
     (name = "nowhere", network = (allow = [])),
-    # the way out: see switchboard.capnp
-    (name = "switchboard", external = (address = "unix:switchboard.sock", http = (style = proxy))),
+    # the way out: one switchboard socket, i.e. one handle (see switchboard.capnp)
+    (name = "switchboard", external = (address = "unix:switchboard.sock", http = ())),
     (name = "state", disk = (path = "state", writable = true)),
   ],
   sockets = [
@@ -24,7 +24,11 @@ const config :Workerd.Config = (
 
 const supervisor :Workerd.Worker = (
   compatibilityDate = "2026-10-01",
-  modules = [(name = "supervisor.js", esModule = embed "supervisor.js")],
+  modules = [
+    (name = "supervisor.js", esModule = embed "supervisor.js"),
+    (name = "labels.js", esModule = embed "labels.js"),
+    (name = "capnweb", esModule = embed "node_modules/capnweb/dist/index-workers.js"),
+  ],
   durableObjectNamespaces = [(className = "Vat", uniqueKey = "toybox-vat", enableSql = true)],
   durableObjectStorage = (localDisk = "state"),
   bindings = [