render3d: the height field and photograph on the CPU as tiles read from a file (off until the game asks)

terrain_tiles_to(dir, key) before a map is made: once made, its heights, photograph and baked normals
are written tile by tile (64 m, TT_TEX) to <dir>/<key>.tiles - fresh every time, header last, so no
other generator's or a torn file is ever read - and the whole copies go. Every read goes through the
tile: resident, else read from the file there and then into a 2048-tile clock, so terrain_height,
terrain_height_smooth, terrain_ortho* and terrain_chunk_heights answer exactly what the whole copy
did (R3D_TT_CHECK: worst 0.0 m over 4000 points) whatever is resident - two machines and the boot's
placements agree to the bit. terrain_chunk_heights takes render3d_st mut for it.

terrain_height_near(read-only): the tile if it is in, else a coarse 2048^2 level (worst 0.91 m), never
the file - for line checks that must not take render3d_st mut. terrain_texel() is the texel size,
terrain_tiles_prefetch(x, z, r, budget) reads ahead, and a frame that reads more than 8 tiles says so
once. R3D_TERRAIN_TILES=<dir> turns it on for a run.

At the overlook with MallocLargeCache=0: 1731 MB off, 1658 MB on; 192 MB written in ~200 ms; the
frame unchanged (0 pixels over 8).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.render3d/r3d.ludic

@@ -25,6 +25,8 @@ import "sky.ludic"
 import "daylight.ludic"
 import "terrain.ludic"
 import "terrain_chunks.ludic"
+import "terrain_tiles.ludic"
+import "terrain_tiles_cut.ludic"
 import "overlay.ludic"
 import "shadow.ludic"
 import "post.ludic"