K19 (caps in list): mesh-broker's list answers the sum of the caps and the limits

Builds: design 47 G §661, §547

Claude-Session: https://claude.ai/code/session_01JG73KEq4zoDfM9WtYpcQgZ

seats/mesh-broker/fixtures/verbs.json

@@ -114,5 +114,27 @@
         }
       ]
     }
+  },
+  {
+    "name": "list answers the sum of the caps and both limits (design 47 G 661)",
+    "of": "verb:list.answer",
+    "given": {
+      "entries": [],
+      "caps": {
+        "sum-mib": 512,
+        "account-limit-mib": 1024,
+        "server-limit-mib": 2048
+      }
+    }
+  },
+  {
+    "name": "list answers a sum with no limit, which is unlimited",
+    "of": "verb:list.answer",
+    "given": {
+      "entries": [],
+      "caps": {
+        "sum-mib": 64
+      }
+    }
   }
 ]

seats/mesh-broker/seat.json

@@ -32,6 +32,27 @@
               "$ref": "#/defs/Held"
             },
             "description": "every entry"
+          },
+          "caps": {
+            "type": "object",
+            "description": "the sum of the bus's caps against its limits, in MiB; an absent limit is unlimited (design 47 G 547, 661)",
+            "properties": {
+              "sum-mib": {
+                "type": "integer",
+                "minimum": 0
+              },
+              "account-limit-mib": {
+                "type": "integer",
+                "minimum": 0
+              },
+              "server-limit-mib": {
+                "type": "integer",
+                "minimum": 0
+              }
+            },
+            "required": [
+              "sum-mib"
+            ]
           }
         },
         "required": [