Initial project: XIAO ESP32S3 + SX1262 LoRa sensor link hytte–hjem

- cabin_node: subscribes to local MQTT (ESPHome sensors), sends
  temperature, battery voltage/SOC and switch states over LoRa SF12
- cabin_gw: receives LoRa packets, publishes JSON to home MQTT broker
- Bidirectional: gateway forwards ON/OFF commands from home HA to node
- cabin_node always in RX mode (12V powered) — commands arrive instantly
- ESPHome config for ESP32 on teknisk rom: Victron MPPT BLE + JBD BMS
  BLE + DS18B20 temperatures + GPIO switches for varme/VVB

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

include/packets.h

@@ -0,0 +1,36 @@
+#pragma once
+#include <stdint.h>
+
+#define CABIN_PACKET_MAGIC 0xCA
+#define CABIN_CMD_MAGIC    0xCB
+
+// Command IDs
+#define CMD_VARME   0x01
+#define CMD_VVB     0x02   // varmtvannsbereder
+
+// Wire format: 13 bytes total
+// At SF12 BW125: ~1.3s airtime
+// flags bits: 0=varme, 1=vvb (1=on, 0=off)
+#define CABIN_FLAG_VARME  (1 << 0)
+#define CABIN_FLAG_VVB    (1 << 1)
+
+struct __attribute__((packed)) CabinPacket {
+  uint8_t  magic;         // = CABIN_PACKET_MAGIC
+  uint8_t  seq;           // rolling counter
+  int16_t  temp_in_x10;   // indoor temp * 10  (213 = 21.3 °C), INT16_MIN = no data
+  int16_t  temp_out_x10;  // outdoor temp * 10, INT16_MIN = no data
+  uint16_t batt_mv;       // battery voltage mV (12V system: ~10000–14400)
+  uint8_t  batt_pct;      // SOC 0–100, 0xFF = no data
+  uint8_t  flags;         // switch states: bit0=varme, bit1=vvb
+  uint32_t sig;           // truncated HMAC-SHA256
+};
+
+// Command packet: home → gateway → node → ESPHome switch
+// 8 bytes total
+struct __attribute__((packed)) CabinCmdPacket {
+  uint8_t  magic;   // = CABIN_CMD_MAGIC
+  uint8_t  seq;
+  uint8_t  cmd_id;  // CMD_VARME, CMD_VVB, ...
+  uint8_t  value;   // 0 = off, 1 = on
+  uint32_t sig;
+};