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

README.md

@@ -203,14 +203,16 @@ MeterData JSON (sender + MQTT):
   "p2_w": 450.00,
   "p3_w": 0.00,
   "bat_v": 3.92,
-  "bat_pct": 78
+  "bat_pct": 78,
+  "rx_reject": 0,
+  "rx_reject_text": "none"
 }
 ```
 
 ### Binary MeterBatch Payload (LoRa)
 Fixed header (little-endian):
 - `magic` u16 = 0xDDB3
-- `schema` u8 = 1
+- `schema` u8 = 2
 - `flags` u8 = 0x01 (bit0 = signed phases)
 - `sender_id` u16 (1..NUM_SENDERS, maps to `EXPECTED_SENDER_IDS`)
 - `batch_id` u16
@@ -222,6 +224,8 @@ Fixed header (little-endian):
 - `err_d` u8 (decode failures, sender-side counter)
 - `err_tx` u8 (LoRa TX failures, sender-side counter)
 - `err_last` u8 (last error code: 0=None, 1=MeterRead, 2=Decode, 3=LoraTx, 4=TimeSync)
+- `err_rx_reject` u8 (last RX reject reason)
+- `err_rx_reject` u8 (last RX reject reason: 0=None, 1=crc_fail, 2=bad_protocol_version, 3=wrong_role, 4=wrong_payload_type, 5=length_mismatch, 6=device_id_mismatch, 7=batch_id_mismatch)
 - MQTT faults payload also includes `err_last_text` (string) and `err_last_age` (seconds).
 
 Body: