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

include/meter_driver.h

@@ -5,3 +5,5 @@
 
 void meter_init();
 bool meter_read(MeterData &data);
+bool meter_poll_frame(const char *&frame, size_t &len);
+bool meter_parse_frame(const char *frame, size_t len, MeterData &data);