- Add setPositionColor(): maps SSI value to HSV hue (0=red, 120=green, 240=blue)
- LED takes the position hue as its resting colour between reads
- Blue during active read unchanged
- All error paths now flash red before printing ERR
Replace dual-core FIFO bit-bang approach with a dedicated PIO program:
- Add ssi.pio: PIO assembly for SSI master (side-set CLK, sample DATA)
- Add ssi.pio.h: pioasm-generated C header
- ssi_pio_init(): load program, wire CLK/DATA pins, set clock divider
- ssi_pio_reconfigure_speed(): live clock-divider update between reads
- ssi_pio_read(): push bit-count to TX FIFO, block on RX result word
- Remove dual-core headers, FIFO structs, setup1/loop1, requestSsiRead
- Bit limit reduced to 1..32 (single PIO ISR word)
- TX_DI pin ownership transferred to PIO (no pinMode/digitalWrite)
