fix: clock polarity on csn falling and rising. Proper wait after csn falling and before csn rising

tests: fix _t meaning (tri state)

fix: short last sck pulse on slower clock

feat: add test for max divisor (256)

feat: implement spi model narrow sck check

fix: assumptions about synthesizable code

feat: add test for lsbfirst

chore: remove todo comments

feat: add csn pulse test and rx, tx disabling test

feat: add tests for rx blocking

chore: move spi models to separate file