Add RX reject reasons to telemetry and UI

BACKWARD-INCOMPATIBLE: MeterBatch schema bumped to v2 with err_rx_reject.
- Track and log RX reject reasons (CRC/protocol/role/payload/length/id/batch)
- Include rx_reject in sender telemetry JSON and receiver web UI
- Add lora_receive reject reason logging under SERIAL_DEBUG_MODE

src/lora_transport.cpp

@@ -3,6 +3,26 @@
 #include <SPI.h>
 #include <math.h>
 
+static RxRejectReason g_last_rx_reject_reason = RxRejectReason::None;
+static uint32_t g_last_rx_reject_log_ms = 0;
+
+static void note_reject(RxRejectReason reason) {
+  g_last_rx_reject_reason = reason;
+  if (SERIAL_DEBUG_MODE) {
+    uint32_t now_ms = millis();
+    if (now_ms - g_last_rx_reject_log_ms >= 1000) {
+      g_last_rx_reject_log_ms = now_ms;
+      Serial.printf("lora_rx: reject reason=%s\n", rx_reject_reason_text(reason));
+    }
+  }
+}
+
+RxRejectReason lora_get_last_rx_reject_reason() {
+  RxRejectReason reason = g_last_rx_reject_reason;
+  g_last_rx_reject_reason = RxRejectReason::None;
+  return reason;
+}
+
 static uint16_t crc16_ccitt(const uint8_t *data, size_t len) {
   uint16_t crc = 0xFFFF;
   for (size_t i = 0; i < len; ++i) {
@@ -95,6 +115,7 @@ bool lora_receive(LoraPacket &pkt, uint32_t timeout_ms) {
         while (LoRa.available()) {
           LoRa.read();
         }
+        note_reject(RxRejectReason::LengthMismatch);
         return false;
       }
 
@@ -105,15 +126,18 @@ bool lora_receive(LoraPacket &pkt, uint32_t timeout_ms) {
       }
 
       if (len < 7) {
+        note_reject(RxRejectReason::LengthMismatch);
         return false;
       }
 
       uint16_t crc_calc = crc16_ccitt(buffer, len - 2);
       uint16_t crc_rx = static_cast<uint16_t>(buffer[len - 2] << 8) | buffer[len - 1];
       if (crc_calc != crc_rx) {
+        note_reject(RxRejectReason::CrcFail);
         return false;
       }
       if (buffer[0] != PROTOCOL_VERSION) {
+        note_reject(RxRejectReason::BadProtocol);
         return false;
       }
 
@@ -123,6 +147,7 @@ bool lora_receive(LoraPacket &pkt, uint32_t timeout_ms) {
       pkt.payload_type = static_cast<PayloadType>(buffer[4]);
       pkt.payload_len = len - 7;
       if (pkt.payload_len > LORA_MAX_PAYLOAD) {
+        note_reject(RxRejectReason::LengthMismatch);
         return false;
       }
       memcpy(pkt.payload, &buffer[5], pkt.payload_len);