Release 2.1.1 — divisor-256 deadzone threshold measured

The last undetermined quantizer input is now measured, and corrects 2.1.0:
the threshold at divisor 256 is 359, not 360, so the deadzone ratio is 7/5
rather than 45/32.

All eight reachable divisors are now measured directly; the quantizer contains
no extrapolated value.