nats: the module, and an image that reloads in place

Step 1.1 and 1.2 of novox/hq ADR 0116. The server is a built artifact rather
than the upstream image directly, because it needs an entrypoint of its own:
the host can only recreate a container, and recreating the bus for every
permission change drops every connection and every in-flight ack. nats-server
reloads on SIGHUP by itself, so the config is mounted as a directory (not
digest-tracked, hq issue 103) and the entrypoint watches the one file.

Verified against the real server, not assumed: a user added to the config
connects, a revoked one is refused, both within one poll interval, with the
container's PID and restart count unchanged and "Reloaded: accounts" in its
log.

Two corrections found by checking rather than reading:
- the seat delivers nothing now (hq ADR 0117), and the controller's parser
  refused the manifest until it did — "nats claims mesh-broker, whose holder
  answers for amqp, and nats does not provide amqp"
- pinned to the multi-arch index digest; the first pin was the amd64
  manifest, which builds here and fails on any other architecture

modules/nats/module.json

@@ -0,0 +1,71 @@
+{
+  "module": "nats",
+  "version": "1",
+  "provides": [],
+  "claims": [
+    {
+      "name": "mesh-broker",
+      "scope": "mesh"
+    }
+  ],
+  "capabilities": [
+    "container-runtime"
+  ],
+  "emits": [],
+  "consumes": [],
+  "listens": [
+    {
+      "port": 4222,
+      "protocol": "tcp",
+      "from": "mesh",
+      "why": "the mesh bus \u2014 every link the mesh has, over TLS, reached across the overlay"
+    }
+  ],
+  "guards": [
+    8222
+  ],
+  "resources": [
+    {
+      "id": "jetstream-data",
+      "type": "directory",
+      "path": "/var/lib/mesh-broker-nats",
+      "mode": "0700"
+    },
+    {
+      "id": "conf-dir",
+      "type": "directory",
+      "path": "/var/lib/nats-module/conf",
+      "mode": "0700"
+    },
+    {
+      "id": "server",
+      "type": "container",
+      "name": "mesh-broker-nats",
+      "ports": [
+        "4222:4222",
+        "127.0.0.1:8222:8222"
+      ],
+      "volumes": [
+        "/var/lib/mesh-broker-nats:/data",
+        "/var/lib/nats-module/conf:/etc/nats:ro",
+        "/var/lib/mesh-broker-nats-tls:/tls:ro"
+      ],
+      "artifact": "server"
+    }
+  ],
+  "accesses": [
+    {
+      "path": "/var/lib/mesh-broker-nats-tls",
+      "mode": "read"
+    }
+  ],
+  "build": {
+    "artifacts": [
+      {
+        "name": "server",
+        "kind": "image",
+        "from": "Dockerfile"
+      }
+    ]
+  }
+}