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/crypto.h

@@ -0,0 +1,20 @@
+#pragma once
+#include <stdint.h>
+#include <stddef.h>
+#include <string.h>
+#include "mbedtls/md.h"
+#include "config.h"
+
+// Returns a 4-byte truncated HMAC-SHA256 over `data` of `len` bytes.
+inline uint32_t hmac_sig(const uint8_t *data, size_t len)
+{
+  static const char *key = HMAC_KEY;
+  uint8_t out[32];
+  mbedtls_md_hmac(
+    mbedtls_md_info_from_type(MBEDTLS_MD_SHA256),
+    reinterpret_cast<const uint8_t *>(key), strlen(key),
+    data, len, out);
+  uint32_t sig;
+  memcpy(&sig, out, 4);
+  return sig;
+}