Functions
	sphere_sdf (rfrac=0.3)

	run (vmg, dt, fx, n_steps, vlevels=3, vcycles=10, to_steady=False)

	main ()

Variables
int	N = 32

float	NU = 0.1

Detailed Description

Validation of the upwind-convective velocity-multigrid coarse operator (task #56, Phase 4 of
doc/velocity_mg_plan.md). When implicit-FOU advection is on, the velocity (momentum) solve is an
advection-dominated, non-symmetric Helmholtz system (I - nu*dt*Lap + dt*FOU(u^k)). The opt-in
upwind-convective vel-MG builds the coarse operators as anisotropic const-coeff diffusion + a coarse
first-order-upwind advection from the restricted advecting velocity, keeping every level an M-matrix.

This checks, on a sphere in a periodic box (full 3-D NS + cut-cell IBM + cut-cell pressure, one GPU):
  (1) the vel-MG path stays finite/bounded at high Re where the operator is advection-dominated;
  (2) at steady state it matches the RB-GS implicit-FOU solve (same operator, MG just sets the rate)
      -> drag-equivalent: U_max and U_mean agree to a tight tolerance.
The fine residual + smoother guarantee the exact fine (sharp-IBM) answer regardless of the coarse op,
so MG-on must converge to the same field as RB-GS.