C12: mesh-watchdogs says machine-away and machine-back

Subject parts name the machine, not the publisher; kept 90 days and 16 MiB.

Builds: design 47 G §389, §436

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

health-and-repair/seats/mesh-watchdogs/fixtures/events.json

@@ -0,0 +1,24 @@
+[
+  {
+    "name": "a machine's presence expired",
+    "of": "event:machine-away",
+    "subject": "mesh.seat.mesh-watchdogs.event.machine-away.g14",
+    "given": {
+      "v": 1,
+      "machine": "g14",
+      "at": "2026-11-11T10:00:00Z",
+      "last-heard": "2026-11-11T09:58:00Z"
+    }
+  },
+  {
+    "name": "a machine heard again",
+    "of": "event:machine-back",
+    "subject": "mesh.seat.mesh-watchdogs.event.machine-back.g14",
+    "given": {
+      "v": 1,
+      "machine": "g14",
+      "at": "2026-11-11T10:30:00Z",
+      "last-heard": "2026-11-11T10:30:00Z"
+    }
+  }
+]

health-and-repair/seats/mesh-watchdogs/seat.json

@@ -159,6 +159,82 @@
       }
     }
   ],
+  "events": [
+    {
+      "name": "machine-away",
+      "subject": "machine-away.<machine>",
+      "keep": {
+        "max-age-days": 90,
+        "max-mib": 16
+      },
+      "body": {
+        "type": "object",
+        "description": "the watchman's presence key of a machine expired (ADR 0564 4); published by the holder of this seat alone for another machine (design 47 G 436)",
+        "properties": {
+          "v": {
+            "type": "integer",
+            "description": "the layout's version"
+          },
+          "machine": {
+            "type": "string",
+            "description": "the machine whose presence changed"
+          },
+          "at": {
+            "type": "string",
+            "format": "date-time"
+          },
+          "last-heard": {
+            "type": "string",
+            "format": "date-time",
+            "description": "when the machine was last heard"
+          }
+        },
+        "required": [
+          "v",
+          "machine",
+          "at",
+          "last-heard"
+        ]
+      }
+    },
+    {
+      "name": "machine-back",
+      "subject": "machine-back.<machine>",
+      "keep": {
+        "max-age-days": 90,
+        "max-mib": 16
+      },
+      "body": {
+        "type": "object",
+        "description": "a machine is heard again after it was away",
+        "properties": {
+          "v": {
+            "type": "integer",
+            "description": "the layout's version"
+          },
+          "machine": {
+            "type": "string",
+            "description": "the machine whose presence changed"
+          },
+          "at": {
+            "type": "string",
+            "format": "date-time"
+          },
+          "last-heard": {
+            "type": "string",
+            "format": "date-time",
+            "description": "when the machine was last heard"
+          }
+        },
+        "required": [
+          "v",
+          "machine",
+          "at",
+          "last-heard"
+        ]
+      }
+    }
+  ],
   "states": [
     {
       "name": "presence",