Declare every module's data; the backup holder measures it (hq ADR 0233)

Backup lines are derived from each module's data section instead of written by hand; the holder
measures declared items, reads the array under them, and deletes a retired item only after a last
restore point; the Go providers say each held consumer's size so an empty replacement is seen.

modules/distribution/module.json

@@ -36,6 +36,17 @@
       "why": "every machine pulls images and artifacts from here"
     }
   ],
+  "data": {
+    "own": [
+      {
+        "id": "registry",
+        "path": "${dir:registry-data}",
+        "class": "rebuildable",
+        "backup": "none",
+        "why": "the artifact store: every image is built again from its source, and too large to copy every night (ADR 0214 left it out)"
+      }
+    ]
+  },
   "resources": [
     {
       "id": "state",