feat(routing): Lab Host Router closes the Gateway-to-Lab cross-VM gap

Adds platform/lab-router/ — a narrow service that runs on the Lab Host,
authenticates the Gateway via mTLS, and resolves an opaque routing token to
a local container purely from Docker label state (no Postgres access, no
callback to the Provisioner). Replaces the Gateway's env.internal_ip
assumption (only valid when Gateway and Lab share a Docker daemon) with a
data-driven direct/router decision per lab_host
(lab_hosts.routing_mode/router_endpoint, migration 004), resolved in one
place (platform/gateway/src/lab-endpoint.js's labTarget()) that every
proxy/fetch call site in server.js now goes through.

Extends the existing Docker TLS CA (infra/scripts/docker-tls/generate-docker-tls.sh)
with a second mTLS leaf pair (router-server/gateway-client) for this new
data-plane connection, deployed by the docker-tls Ansible role into a
separate directory from the control-plane pair. Firewall
(rn-lab-host-01.rules) replaces the old speculative wide port-range
allowance with one narrow rule: platform_ip -> the Router's port only.

dev-local stays routing_mode: direct — zero behavior change for local dev,
confirmed by the full existing regression suite passing unmodified.

infra/docker/lab-host/docker-compose.production.yml

@@ -1,10 +1,13 @@
 # Production compose for rn-lab-host-01. Deployed by infra/ansible/roles/lab-host/ to
 # /opt/rootnode/lab-host/docker-compose.yml, run as a systemd unit
-# (rootnode-lab-host.service.j2). Runs exactly one long-lived service: the shared egress proxy
-# every student Lab's outbound traffic passes through (platform/egress-proxy/, unchanged since
-# Phase 1). Student Lab containers themselves are NOT part of this compose file — they're
-# created/destroyed individually by the Provisioner over the remote Docker TLS endpoint
-# (dockerd itself, not compose), same mechanism as dev, just remote.
+# (rootnode-lab-host.service.j2). Runs the shared egress proxy every student Lab's outbound
+# traffic passes through (platform/egress-proxy/, unchanged since Phase 1) and, as of Phase
+# 4.1, the Lab Host Router (platform/lab-router/) that gives the Gateway on rn-platform-01 a
+# way to reach a running Lab without ever touching a Docker-internal address — see
+# docs/PHASE4_1_PREPRODUCTION_HARDENING.md §1-3. Student Lab containers themselves are NOT
+# part of this compose file — they're created/destroyed individually by the Provisioner over
+# the remote Docker TLS endpoint (dockerd itself, not compose), same mechanism as dev, just
+# remote; the Router finds them by label, purely from local Docker state (see its own README).
 
 networks:
   rn-egress:
@@ -22,6 +25,26 @@ services:
                        # is the actual control, see platform/egress-proxy/allowlist.txt)
     networks: [rn-egress]
 
+  lab-router:
+    image: "{{ registry }}/rootnode/lab-router:{{ image_tag }}"
+    restart: unless-stopped
+    # Needs the host's own routing table to reach whichever per-Lab bridge network
+    # (rootnode-lab-net-<id>) the Provisioner created for a given container — those networks
+    # aren't known ahead of time and aren't attachable one-by-one, the same reason egress-proxy
+    # would need this too if it initiated connections outward instead of just accepting them.
+    # Restricted to platform_ip on port {{ lab_router_port | default('8443') }} at the host
+    # firewall (infra/firewall/rn-lab-host-01.rules) — this is the ONLY thing that makes host
+    # networking here safe, same defense-in-depth pattern as the Docker TLS port.
+    network_mode: host
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock:ro
+      - /etc/rootnode/lab-router-tls:/tls:ro
+    environment:
+      LAB_ROUTER_PORT: "{{ lab_router_port | default('8443') }}"
+      LAB_ROUTER_TLS_CA: /tls/ca.pem
+      LAB_ROUTER_TLS_CERT: /tls/router-server-cert.pem
+      LAB_ROUTER_TLS_KEY: /tls/router-server-key.pem
+
   node-exporter:
     image: prom/node-exporter:latest
     restart: unless-stopped