Define the envelope, topic namespace and OBD-II signal registry

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013kj58Jugkic783b8ZrtTVZ

schema/envelope.json

@@ -0,0 +1,51 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://yukart/schema/envelope.json",
+  "title": "Signal envelope",
+  "description": "Every decoded signal published on yukart/vehicle/* and yukart/gnss/*. Raw CAN uses the binary frame encoding instead.",
+  "type": "object",
+  "required": [
+    "timestamp",
+    "monotonic",
+    "value",
+    "unit"
+  ],
+  "additionalProperties": false,
+  "properties": {
+    "timestamp": {
+      "type": "integer",
+      "description": "UTC epoch microseconds. Absolute reference. Subject to NTP steps; use monotonic for ordering."
+    },
+    "monotonic": {
+      "type": "integer",
+      "description": "Nanoseconds from CLOCK_MONOTONIC. Authoritative for ordering and for any interval within a session. Not comparable across boots."
+    },
+    "value": {
+      "type": [
+        "number",
+        "string",
+        "boolean",
+        "null"
+      ],
+      "description": "null means the signal is known but currently has no value."
+    },
+    "unit": {
+      "type": "string",
+      "description": "Derived from signals.json. Never authored per message."
+    },
+    "quality": {
+      "enum": [
+        "ok",
+        "stale",
+        "invalid"
+      ],
+      "default": "ok",
+      "description": "Omitted when ok."
+    },
+    "mock": {
+      "type": "boolean",
+      "default": false,
+      "description": "Present and true when synthesised. Must be rendered visibly by any display."
+    }
+  }
+}