feat(render3d): terrain_chunk_heights - the ground's B-spline heights a chunk at a time (23.5)

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>

packages/ludic.render3d/r3d.ludic

@@ -23,6 +23,7 @@ import "camera.ludic"
 import "sky.ludic"
 import "daylight.ludic"
 import "terrain.ludic"
+import "terrain_chunks.ludic"
 import "overlay.ludic"
 import "shadow.ludic"
 import "post.ludic"