render3d: the terrain's GPU maps paged round the camera while the tiles are on

At the cut the whole 4096 height, normal and photograph textures go, replaced by a coarse
2048 level (the CPU's tt_coarse uploaded; the normal and photograph blitted down on the GPU,
the photograph mipmapped) and a pool of fine tiles in three array textures - heights, normals,
photograph, each layer a tile with a one-texel border - addressed through a tt_n^2 page table
(u_tp_page: slot + 1, 0 = the coarse level). The pool holds the tiles within the reach (900 m,
or the fog wall's when nearer) and a ring, and is made again when the fog wall changes its size
(terrain_pages_fog). Once a frame (tp_frame, beside tt_frame) tiles past the reach and two tiles
go and the wanted ones come in nearest first, 8 a frame, written into a staging buffer kept for
the process (two halves, one per frame in flight) and copied into their layers inside the frame's
own command buffer - no submit of their own - with the page table re-uploaded only when it
changed. tp_bind binds the pool (or 1-layer stand-in arrays while paging is off, u_tp_on = 0) for
every program that reads the ground: terrain_bind_height (models, scatter, shadow_bind, grass),
terrain_bind_prog, the sun pass and the shadow bake. grass_cull.comp reads through the same page
table. R3D_VKMEM prints the pool's line. Tiles off, nothing changes. Compile-only: not run.

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

packages/ludic.render3d/terrain_tiles_cut.ludic

@@ -35,6 +35,7 @@ function terrain_tiles_cut(render3d_st: mut Render3dState) -> void {
   if r3d_env_has(render3d_st, "R3D_TT_CHECK") { tt_check(render3d_st) }
   free(render3d_st.ter_heights);  render3d_st.ter_heights = null
   free(render3d_st.ter_ortho_px);  render3d_st.ter_ortho_px = null
+  tp_cut(render3d_st)
   tt_say_cut(path, TT_HEAD + n * n * (TT_TEX * TT_TEX * 2 + side * side) * 4, (gl_now_us() - t0) / 1000)
 }