Make IEC 62056-21 meter input non-blocking

- Add RX state machine with frame buffer, timeouts, and debug counters

- Expose meter_poll_frame/meter_parse_frame and reuse existing OBIS parsing

- Use cached last-valid frame at 1 Hz sampling to avoid blocking

- Document non-blocking meter handling in README

README.md

@@ -54,6 +54,7 @@ Variants:
   - Energy total: 1-0:1.8.0*255
   - Total power: 1-0:16.7.0*255
   - Phase power: 36.7 / 56.7 / 76.7
+- Meter input is parsed via a non-blocking RX state machine; the last valid frame is reused for 1 Hz sampling.
 - Reads battery voltage and estimates SoC.
 - Builds compact binary batch payload, wraps in LoRa packet, transmits.
 - Light sleeps between meter reads; batches are sent every 30s.