resampled_velocity imports onto any single-block grid (experimental):
- interpolation: linear (the default off a periodic box) or cubic reads samples on rectilinear source axes, from layout.coordinates or a uniform box_origin + box_length, and interpolates them to the case's cell centres, on curvilinear, rotated, stretched and wall-bounded grids; periodic names the source axes to wrap;
- the field is staged as Ucat and the runtime makes it divergence-free with the case's own pressure solve (projection: picurv_discrete);
- the Fourier path for triply periodic boxes is unchanged.