docs: clarify controller roles and load switching boundaries

docs/HARDWARE.md

@@ -158,3 +158,30 @@ Current Cargo ESP32 GPIO plan:
 - GPIO 21: SSD1306 OLED I2C SDA
 - GPIO 22: SSD1306 OLED I2C SCL
 - GPIO 25: OLED setup/status button
+
+## Automotive Load-Switching Layer
+
+Current v1 load-control assumption:
+
+    Cargo ESP32
+        -> control outputs
+        -> automotive load-switching layer
+        -> fused loads
+
+Expected fused loads:
+
+- Fridge
+- Starlink
+- Lights
+- Spare output
+
+The ESP32 relay board is treated as a trigger/control layer only. Major load current must be handled by properly fused automotive wiring, relays, contactors, smart high-side switches, or another suitable fused distribution module.
+
+The exact load-switching hardware is not finalized yet.
+
+Candidate options:
+
+- Bosch-style automotive relays
+- WUPP-style fused relay/distribution module
+- Smart high-side switching
+- Future custom PCB