Refactor ESP-NOW air protocol to nanopb protobuf.

Add esp_now_messages.proto with EspNowMessage types, encode/decode helpers,
and Makefile targets to regenerate firmware and UART schemas together.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/esp_now_proto.c

@@ -0,0 +1,94 @@
+#include "esp_now_proto.h"
+#include "client_registry.h"
+#include "pb_decode.h"
+#include "pb_encode.h"
+#include <string.h>
+
+static bool encode_mac(pb_ostream_t *stream, const pb_field_t *field,
+                       void *const *arg) {
+  const uint8_t *mac = (const uint8_t *)*arg;
+  if (mac == NULL) {
+    return true;
+  }
+  if (!pb_encode_tag_for_field(stream, field)) {
+    return false;
+  }
+  return pb_encode_string(stream, mac, CLIENT_MAC_LEN);
+}
+
+static bool decode_mac(pb_istream_t *stream, const pb_field_t *field,
+                       void **arg) {
+  (void)field;
+  uint8_t *mac = (uint8_t *)*arg;
+  if (mac == NULL) {
+    return false;
+  }
+  if (stream->bytes_left > CLIENT_MAC_LEN) {
+    return false;
+  }
+  return pb_read(stream, mac, stream->bytes_left);
+}
+
+esp_err_t esp_now_proto_encode(const alox_EspNowMessage *msg, uint8_t *buf,
+                               size_t buf_size, size_t *out_len) {
+  if (msg == NULL || buf == NULL || out_len == NULL || buf_size == 0) {
+    return ESP_ERR_INVALID_ARG;
+  }
+
+  pb_ostream_t stream = pb_ostream_from_buffer(buf, buf_size);
+  if (!pb_encode(&stream, alox_EspNowMessage_fields, msg)) {
+    return ESP_FAIL;
+  }
+
+  *out_len = stream.bytes_written;
+  return ESP_OK;
+}
+
+void esp_now_proto_setup_presence_encode(alox_EspNowSlavePresence *presence,
+                                         const uint8_t mac[6]) {
+  if (presence == NULL) {
+    return;
+  }
+  presence->mac.funcs.encode = encode_mac;
+  presence->mac.arg = (void *)mac;
+}
+
+void esp_now_proto_setup_message_decode(alox_EspNowMessage *msg,
+                                        uint8_t mac_out[6]) {
+  if (msg == NULL || mac_out == NULL) {
+    return;
+  }
+  msg->payload.slave_info.mac.funcs.decode = decode_mac;
+  msg->payload.slave_info.mac.arg = mac_out;
+  msg->payload.heartbeat.mac.funcs.decode = decode_mac;
+  msg->payload.heartbeat.mac.arg = mac_out;
+}
+
+esp_err_t esp_now_proto_decode(const uint8_t *data, size_t len,
+                               alox_EspNowMessage *msg) {
+  if (data == NULL || msg == NULL || len == 0) {
+    return ESP_ERR_INVALID_ARG;
+  }
+
+  pb_istream_t stream = pb_istream_from_buffer(data, len);
+  if (!pb_decode(&stream, alox_EspNowMessage_fields, msg)) {
+    return ESP_FAIL;
+  }
+  return ESP_OK;
+}
+
+const alox_EspNowSlavePresence *
+esp_now_proto_get_presence(const alox_EspNowMessage *msg) {
+  if (msg == NULL) {
+    return NULL;
+  }
+
+  switch (msg->which_payload) {
+  case alox_EspNowMessage_slave_info_tag:
+    return &msg->payload.slave_info;
+  case alox_EspNowMessage_heartbeat_tag:
+    return &msg->payload.heartbeat;
+  default:
+    return NULL;
+  }
+}