Boundary v0: WebSocket with heartbeats, acked commands and mock state

One socket on :7700 carries hello, heartbeats both ways (5 s timeout), 5 Hz
state snapshots of envelopes, and idempotent commands. 'mark' is forwarded to
the recorder's control socket. --mock synthesises degrading state flagged as
mock in every envelope; without it a client gets hello and heartbeats only.
Peers outside loopback, RFC 1918 and the tailnet are refused. Deployed on the
car Orin as yukart-gw.service.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

deploy/yukart-gw.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Yukart gateway: the display boundary (svc-gw)
+Documentation=gitea:yukart/svc-gw
+StartLimitIntervalSec=0
+
+[Service]
+User=mikkeli
+# Add --mock only on a bench unit: every message then says so, and displays must show it.
+ExecStart=/usr/local/bin/yukart-gw --listen 0.0.0.0:7700
+Restart=always
+RestartSec=2
+
+[Install]
+WantedBy=multi-user.target