Functions
	packing_sdf (N, radius_frac=0.18)

	run (SolverCls, N, mu=0.1, dt=60.0, F=1e-3, steps=400)

	main ()

Detailed Description

Phase-4 verification (collocated grid): Stokes flow through a periodic 2x2x2 sphere packing with the
Robust-Scaled cut-cell IBM — the first immersed-solid test of the collocated solver. A body force drives
flow through the pores; the cut-cell overlay enforces no-slip at the cell-centered velocities and the
shared cut-cell pressure operator + approximate (MAC) projection enforce incompressibility.

This is the §4 gate of doc/sdflow_colocated_plan.md: the collocated permeability must (a) be incompressible
(projected FACE field divergence-free to machine precision), (b) satisfy exact no-slip in the deep solid,
and (c) converge — across resolution AND toward the staggered solver, which is itself validated against the
Zick & Homsy sphere-array drag. The plain area-weighted face averaging (Option A) + central-difference cell
correction are tested here; escalation to the open-centroid reconstruction (Option B) / openness-aware cell
gradient is only warranted if this gate fails.