The uplink's managers are modules (hq ADR 0117)

networkmanager, systemd-networkd and dhcpcd each claim the-uplink and
declare only what keeps the machine's own network manager from
contradicting the mesh: the resolver file left to resolv-conf, mesh0
left alone. Never a link, profile or credential — the link is the
mesh's only channel to the machine, so NetworkManager and networkd are
reloaded on a change, never restarted, and dhcpcd (no reload; a restart
drops the address) takes its block at its next start.

modules/systemd-networkd/module.json

@@ -0,0 +1,38 @@
+{
+  "module": "systemd-networkd",
+  "version": "1",
+  "capabilities": [
+    "package-manager",
+    "service-manager"
+  ],
+  "claims": [
+    {
+      "name": "the-uplink",
+      "scope": "node"
+    }
+  ],
+  "resources": [
+    {
+      "id": "package",
+      "type": "package",
+      "package": "systemd"
+    },
+    {
+      "id": "config",
+      "type": "file",
+      "path": "/etc/systemd/network/00-mesh0.network",
+      "mode": "0644",
+      "content": "# Managed by the mesh (module systemd-networkd). Replaced on every push; edit\n# the catalogue instead.\n#\n# This machine's uplink is systemd-networkd's, and the mesh asks one thing of it\n# here (novox/hq ADR 0117): leave the private network's interface alone. mesh0\n# is the mesh's; the mesh brings it up and configures it itself. The mesh never\n# declares a link, an address, a route, a wireless network or its credentials,\n# nor a network file for any of this machine's own interfaces \u2014 those are\n# the operator's, and the link they make is the only channel the mesh reaches\n# this machine over.\n#\n# 00-: networkd applies the first .network file, in alphanumeric order across\n# every directory, that matches an interface, and ignores every later one even\n# if it matches too (systemd.network(5), [Match]). A catch-all of the operator's\n# \u2014 Name=*, Type=ether, a file with no [Match] at all \u2014 sorted before\n# this one would claim mesh0 first. 00 sorts before every numbered prefix the\n# man page recommends.\n#\n# Unmanaged=yes: \"no attempts are made to bring up or configure matching links,\n# equivalent to when there are no matching network files\" (systemd.network(5),\n# [Link], since 233). A match that ends the search, and does nothing else.\n#\n# No DNS setting, because none is needed: networkd never writes\n# /etc/resolv.conf. What it learns from a lease it hands only to\n# systemd-resolved, and the resolver file stays whatever resolv-conf wrote.\n# Whether resolved runs, and what it does with that, is the resolver\n# configuration's question, not the uplink's.\n#\n# The service is reloaded when this file changes, never restarted: a restart\n# drops the links networkd holds, this machine's channel to the mesh among them.\n[Match]\nName=mesh0\n\n[Link]\nUnmanaged=yes\n"
+    },
+    {
+      "id": "service",
+      "type": "service",
+      "unit": "systemd-networkd.service",
+      "state": "running",
+      "boot": "enabled",
+      "reload-on": [
+        "config"
+      ]
+    }
+  ]
+}