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

docs/GUIDELINES.md

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+---
+type: guidelines
+---
+# Contract guidelines
+
+## 1. `signals.json` is the single source for a signal's identity
+- Topic name and the envelope's `unit` are both **derived** from a registry entry.
+  Neither is authored by hand anywhere else.  
+- ⚠ **The unit appears in both the topic name and the payload, so they can
+  contradict each other.** Generating both from one entry is what stops that; a
+  hand-written `"unit"` reintroduces the failure it was meant to prevent.
+- Signal names carry their unit (`vehicle_speed_kph`, `coolant_temp_c`), so a unit
+  mismatch is visible at the call site rather than one registry lookup away.
+
+## 2. Two clocks, and they answer different questions
+- `monotonic` is authoritative for **ordering and intervals**. It cannot step.  
+- `timestamp` is the **absolute reference** and can step: the car boots with no
+  network, LTE connects several minutes in, and NTP corrects the clock mid-drive.  
+- ⚠ **Never derive a duration from `timestamp`.** A step lands inside the interval
+  and the result is silently wrong, most likely during exactly the drive worth
+  keeping. Durations come from `monotonic`.
+
+## 3. Raw CAN is not the envelope
+- `yukart/can/raw` is binary. A four-key JSON object at 1000–3000 frames/sec is
+  not a reasonable encoding.
+- The envelope is for decoded signals, which are low-rate by comparison.
+
+## 4. Absent means default
+- `quality` is omitted when `ok`; `mock` is omitted when false. A consumer treats
+  absence as the default, never as an error.
+
+## 5. This repo is versioned by git tag
+- 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