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/systemd/toybox-switchboard.service

@@ -1,12 +1,14 @@
 [Unit]
-Description=toybox switchboard (workerd forward proxy)
+Description=toybox switchboard (workerd, Cap'n Web egress handles)
 Requires=toybox-switchboard.socket
 
 [Service]
-ExecStart=/opt/toybox/node_modules/workerd/bin/workerd serve --experimental /opt/toybox/switchboard.capnp -S http=3
+# the local (localhost) handle goes where no other unit can open it;
+# hand it out deliberately, if ever
+ExecStart=/opt/toybox/node_modules/workerd/bin/workerd serve --experimental /opt/toybox/switchboard.capnp -S public=3 -s local=unix:/run/toybox-switchboard/local.sock
+RuntimeDirectory=toybox-switchboard
+RuntimeDirectoryMode=0700
 DynamicUser=yes
-# the outer fence: switchboard.capnp allows "local" for tests; deny it here
-IPAddressDeny=localhost link-local multicast
 ProtectSystem=strict
 ProtectHome=yes
 PrivateTmp=yes