docs: prepare api contract for lilygo transition

docs/ARCHITECTURE.md

@@ -277,3 +277,20 @@ The Cargo ESP32 is the source of truth for:
 - 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.
+
+## Cargo ESP Hardware Transition Plan
+
+Current active development and testing remains on the existing 2-channel relay board.
+
+A LILYGO T-Relay-S3 6-way board has been ordered as a future Cargo ESP candidate.
+
+Transition strategy:
+
+- Keep current 2-channel board working.
+- Make the Cargo ESP API output-count agnostic.
+- Represent outputs as a list of output objects.
+- Let clients render whatever output list the Cargo ESP exposes.
+- Add a hardware profile/config layer before migrating firmware to LilyGO.
+- Bench test LilyGO relay boot behavior before connecting it to the WUPP block.
+
+The API should not require redesign when moving from 2 outputs to 6 outputs.