n x n samples of chunk (i, j) of a size_m grid from the terrain's corner, row-major into the
caller's buffer with nothing allocated, each the (1 4 1) / 6 B-spline filter of the texels under it
(ter_spline_at): what ludic.physics' jph_shape_heightfield_bspline makes of the whole map, so a
chunk's physics ground matches the drawn one and its neighbours' to the bit. The read-back lives
for the whole map (terrain_generate to terrain_unload). examples/rendering/chunks.ludic, in the
suite, checks the shared edges, a sample against the filter by hand, and a short buffer refused.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
