F2: a fixed finite stencil does not cover every null direction

The same reasoning permits velocities in $\mathrm{conv}S$. An increasing stencil may avoid this particular obstruction, but requires an actual routing and resource theorem. Isotropy is not proved by the upper bound $\Vert s{\Vert }_2\le 1$ alone.

The outline's En08-B labels prop:exact-null and prop:fixed-stencil are treated as documentary identifiers. The tests in this section are established by their printed arguments, not by importing unsupplied predecessor proofs.