A version prepares its state before it runs

The mesh derives the preparation from the module's own resource instead of each module hand-writing
a step beside it (novox/hq ADR 0135). A manifest says one word — `prepares` — and the mesh runs that
module's own program in its preparation mode, in the module's own context: the same image, the same
environment, the same mounts, because it is the same code. A published port and a fixed address are
taken away rather than copied, since the version being replaced still holds them.

One word for every kind of module: a Go binary receives `prepare` as its argument, a bundle receives
it through the runtime whose entry takes the same word. The control plane answers it like anything
else — its own schema stops being a special case, and its hand-written step is gone.

module.json

@@ -27,6 +27,7 @@
     "bus": "/var/lib/mesh/mesh-controller/bus"
   },
   "secrets-owner": "65534:65534",
+  "prepares": true,
   "resources": [
     {
       "id": "mesh-state",
@@ -34,30 +35,6 @@
       "path": "/var/lib/mesh/mesh-controller",
       "mode": "0700"
     },
-    {
-      "id": "migrate",
-      "type": "container",
-      "name": "mesh-controller-migrate",
-      "run-once": true,
-      "network": "host",
-      "args": [
-        "migrate"
-      ],
-      "env": {
-        "MESH_STORE_INVENTORY_FILE": "/run/secrets/inventory",
-        "MESH_STORE_IDENTITY_FILE": "/run/secrets/identity",
-        "MESH_STORE_LICENCES_FILE": "/run/secrets/licences",
-        "MESH_STORE_INVENTORY_PORT": "${seat:mesh-store:5432}",
-        "MESH_STORE_IDENTITY_PORT": "${seat:mesh-store:5432}",
-        "MESH_STORE_LICENCES_PORT": "${seat:mesh-store:5432}"
-      },
-      "volumes": [
-        "/var/lib/mesh/mesh-controller/inventory:/run/secrets/inventory:ro",
-        "/var/lib/mesh/mesh-controller/identity:/run/secrets/identity:ro",
-        "/var/lib/mesh/mesh-controller/licences:/run/secrets/licences:ro"
-      ],
-      "artifact": "server"
-    },
     {
       "id": "server",
       "type": "container",