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/pnpm-lock.yaml

@@ -7,6 +7,10 @@ settings:
 importers:
 
   .:
+    dependencies:
+      capnweb:
+        specifier: 0.12.0
+        version: 0.12.0
     devDependencies:
       workerd:
         specifier: 1.20261006.1
@@ -47,6 +51,9 @@ packages:
     cpu: [x64]
     os: [win32]
 
+  capnweb@0.12.0:
+    resolution: {integrity: sha512-jgZ/LMtMVTi+RVlooIslH78Gg23XbDTdvcLghWx3oz17D6u5GuJARt+uhZk/wcTo+f81eeabfnow4d3zLBj+JQ==}
+
   workerd@1.20261006.1:
     resolution: {integrity: sha512-8MadBuffZ5NdpLwZjUmsqT5QlaKXq4gjgT6wzcKeynZTICNXyePhc+5QVoKiK7+MMhFTLNrRO2KXLlpk8Q6BkA==}
     engines: {node: '>=16'}
@@ -81,6 +88,8 @@ snapshots:
   '@cloudflare/workerd-windows-64@1.20261006.1':
     optional: true
 
+  capnweb@0.12.0: {}
+
   workerd@1.20261006.1:
     optionalDependencies:
       '@cloudflare/workerd-darwin-64': 1.20261006.1