docs: define cargo esp api contract

docs/ARCHITECTURE.md

@@ -263,3 +263,17 @@ Current responsibility split:
 - Dashboard ESP32-S3 should not wire directly to cargo-area DS18B20 sensors for v1.
 
 This means future dashboard work should display existing Cargo ESP temperature telemetry rather than reimplementing temperature sensing on the dashboard.
+
+## Cargo ESP API Contract
+
+The Cargo ESP API contract is documented in `docs/cargo-api-contract.md`.
+
+The Cargo ESP32 is the source of truth for:
+
+- Relay/output state
+- DS18B20 temperature readings
+- BMS telemetry
+- Alarm/fault state
+- Local configuration
+
+The ESP32-S3 dashboard should consume Cargo ESP API data over WiFi/HTTP REST instead of directly owning cargo sensors or relay state.