BSP 1: one tick across the seam

The control code never touches a register; it goes through the 59 functions of bsp_api.h. Follow the current-loop tick of millisecond 50 across that seam twice: reading, ADC 32908 minus the calibrated offset 32790, times 1.2588 mA, is 0.1485 A; writing, 0.9086 V becomes compare value 3187. Every stop puts the board's real bsp_api.c beside the SIL's bsp_api_virtual.c, with numbers from the real BSP code compiled on a Mac. Last, the contract between the two sides: 0 V is a 45 % duty, the limit is 90 %.