Decoded Z27AG signals: layouts in signals.json can blocks, the only source

engine_rpm moves to source z27ag (no OBD-II on this car's CAN). Ten more:
accelerator, brake, handbrake, indicators, side lights, dipped beam, doors,
A/C compressor. Guideline 6: no service writes a CAN ID of its own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

README.md

@@ -17,11 +17,14 @@ definitions from here.
 |---|---|
 | [`schema/topics.json`](schema/topics.json) | Topic namespace: payload kind, retention, rate, publisher |
 | [`schema/envelope.json`](schema/envelope.json) | JSON Schema for the decoded-signal envelope |
-| [`schema/signals.json`](schema/signals.json) | Signal registry — the single source for name, unit, type and range |
+| [`schema/signals.json`](schema/signals.json) | Signal registry — the single source for name, unit, type and range, and for where each decoded Z27AG signal sits in its CAN frame (`can` block) |
 | [`docs/BOUNDARY.md`](docs/BOUNDARY.md) | Display boundary protocol: what crosses between `svc-gw` and the app |
 | [`checklists/`](checklists/) | Recording checklists as data; the app bundles exact copies (its **Captures** page) |
 
-The Z27AG DBC lands here too, once Phase 2 produces one.
+Decoded Z27AG signals (source `z27ag`) carry their frame layout in `signals.json` itself,
+so name, unit and bits cannot drift apart. A DBC for tools like SavvyCAN or cantools,
+if one is wanted, is generated from it, never edited by hand. Findings behind each
+layout: [`docs/impl/reports/`](docs/impl/reports/).
 
 - For implementation practice guidelines, refer to the [guidelines](docs/GUIDELINES.md);  
 - For implementation plans and history, refer to [them](docs/impl/);

docs/GUIDELINES.md

@@ -33,4 +33,14 @@ type: guidelines
 - There is no version in any topic or payload. A recorded session names the
   contracts tag it was captured under, which is what keeps old captures readable.
 
-## 6. To be added
+## 6. A decoded signal's layout lives only in its registry entry
+- Every `source: z27ag` signal has a `can` block in `signals.json`: frame ID, byte,
+  bit or length, byte order, scale and offset. The decoder reads those blocks and
+  nothing else, so a correction lands in one file and every consumer sees it.
+- ⚠ **No CAN ID or bit position is written in any service's code.** A layout
+  hard-coded downstream is a second source that silently disagrees the day the
+  registry is corrected.
+- Each block names the report with its evidence (`evidence`). A layout without
+  evidence is a guess and does not go in.
+
+## 7. To be added

schema/signals.json

@@ -1,15 +1,24 @@
 {
-  "$comment": "Single source for signal identity. Topic name and envelope unit are both derived from these entries and are never authored separately. source=obd2 is standardised; source=z27ag is reverse-engineered and lives in the DBC. source=host is the Orin itself (orin-status), not the car.",
+  "$comment": "Single source for signal identity. Topic name and envelope unit are both derived from these entries and are never authored separately. source=obd2 is standardised; source=host is the Orin itself (orin-status), not the car. source=z27ag is reverse-engineered from the car's own broadcast frames, and its 'can' block is the only place its layout is written down: id (11-bit, hex string), byte (first data byte, 0-7), length in bits (1, 8 or 16), bit (length 1 only: 0 = LSB of that byte), order (length 16: big = byte first is most significant, little = the reverse), scale and offset (value = raw * scale + offset; booleans are raw != 0). 'evidence' names the contracts report that found it.",
   "signals": {
     "engine_rpm": {
       "unit": "rpm",
       "type": "number",
-      "source": "obd2",
+      "source": "z27ag",
       "description": "Engine speed",
       "range": [
         0,
         8000
-      ]
+      ],
+      "can": {
+        "id": "0x210",
+        "byte": 4,
+        "length": 16,
+        "order": "big",
+        "scale": 1,
+        "offset": 0,
+        "evidence": "Z27AG-DECODE-1"
+      }
     },
     "vehicle_speed_kph": {
       "unit": "kph",
@@ -91,6 +100,141 @@
         656
       ]
     },
+    "accel_pedal_pct": {
+      "unit": "percent",
+      "type": "number",
+      "source": "z27ag",
+      "description": "Accelerator pedal position (0x00 at rest, 0xFA flat)",
+      "range": [
+        0,
+        100
+      ],
+      "can": {
+        "id": "0x210",
+        "byte": 0,
+        "length": 8,
+        "scale": 0.4,
+        "offset": 0,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "brake_pedal": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "Brake pedal switch: pressed",
+      "can": {
+        "id": "0x231",
+        "byte": 4,
+        "bit": 1,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "handbrake": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "Handbrake applied",
+      "can": {
+        "id": "0x239",
+        "byte": 0,
+        "bit": 2,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "indicator_left": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "Left indicator lamp, as it flashes (on and off at the flasher's rate)",
+      "can": {
+        "id": "0x423",
+        "byte": 2,
+        "bit": 6,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "indicator_right": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "Right indicator lamp, as it flashes. Hazards were seen on the left bit only: open",
+      "can": {
+        "id": "0x423",
+        "byte": 2,
+        "bit": 5,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "side_lights": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "Side (position) lights on; also on with dipped beam",
+      "can": {
+        "id": "0x423",
+        "byte": 2,
+        "bit": 2,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "dipped_beam": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "Dipped headlights on",
+      "can": {
+        "id": "0x423",
+        "byte": 3,
+        "bit": 1,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "door_open": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "Any door open",
+      "can": {
+        "id": "0x423",
+        "byte": 2,
+        "bit": 1,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "driver_door_open": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "Driver's door open",
+      "can": {
+        "id": "0x423",
+        "byte": 0,
+        "bit": 4,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
+    "ac_compressor": {
+      "unit": "",
+      "type": "boolean",
+      "source": "z27ag",
+      "description": "A/C compressor engaged (engine running)",
+      "can": {
+        "id": "0x13D",
+        "byte": 6,
+        "bit": 7,
+        "length": 1,
+        "evidence": "Z27AG-DECODE-1"
+      }
+    },
     "orin_cpu_temp_c": {
       "unit": "celsius",
       "type": "number",