Functions
	sphere_sdf (rfrac=0.3)

	run (implicit, dt, fx, n_steps, to_steady=False)

	main ()

Variables
int	N = 32

float	NU = 0.1

Detailed Description

Verification of the implicit-FOU deferred-correction advection (dcfd.set_implicit_advection). The
distributed solver is a full Navier-Stokes solver (Koren TVD advection). The default advection is
EXPLICIT (Picard-lagged) and therefore CFL-limited: at high Reynolds number / large dt it goes unstable.
The implicit-FOU mode solves the first-order-upwind part of advection implicitly (diagonally dominant ->
unconditionally stable for advection) and keeps the (Koren - FOU) correction explicit, so the scheme is
still Koren TVD at convergence but is robust at high Re, matching the production solver's deferred
correction. This checks both:
  (1) high Re / large dt: explicit BLOWS UP, implicit-FOU stays finite and bounded;
  (2) moderate Re (where explicit is stable): the two agree -> same Koren scheme at convergence.
Flow around a sphere in a periodic box (full 3-D NS + cut-cell IBM + cut-cell pressure). One GPU.