STANDARDIZED COMPUTE COST (per operation, amortized)
  charge  N=b^2+ab-a^2         : 158219.9 ps   (3 mul, 2 add; exact int)
  residue r (finite a)         : 330 ns   (1 log + 1 floor + arith)
  residue r (infinite a=2^p)   : 354 ns   (1 log + 1 floor; a never built)
  flash_read = charge + residue: ~330 ns dominated by the single log

  ALL O(1) in the distance k. Cost independent of how far from Omega.
  verified flat: depth 2 and depth 80 both ~330 ns (17-digit vs 1-digit a).
