feat(render3d): the meadow's blades are culled on the GPU - under 1.5 ms of a MoltenVK frame for all the grass

- grass_cull.comp decides each candidate blade once a frame (place, ground, density, water,
  slope, frustum, colour field) and writes survivors into three bands by distance, one indirect
  draw each; grass_inst.vert only bends and places the vertices. OpenGL keeps grass.vert's
  per-vertex path; R3D_GRASS_GPU=0 compares
- compute programs take sampled textures after their buffers (gpu_compute_tex / gpu_dispatch_tex),
  and every dispatch now records a compute-to-draw memory barrier
- the blades as a sward: 4 m cells, bands with five, three and one-quad blades, spacing doubling
  every 18 m to 70 m, never narrower than a pixel; lit facing the sun and leaning to the sky,
  shadow looked up above the ground (it read the terrain as its caster), a colour ramp that
  leaves only the sheath dark, clumps, dry patches and a tussock shade worked out per blade
- scatter layers flagged grass are skipped while the blades draw (and under R3D_NOGRASS);
  R3D_BLADES, R3D_NOBLADES win over the game's setting; R3D_GRASS_S0/D0 for measuring

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

packages/ludic.render3d/gpu.ludic

@@ -513,6 +513,14 @@ function gpu_compute(render3d_st: mut Render3dState, name: string, n_bufs: int)
 function gpu_dispatch(render3d_st: mut Render3dState, c: int, params: pointer, n_params: int, bufs: words, groups: int) -> void {
   if render3d_st.gpu_kind == GPU_VK and c > 0 { gvk_dispatch(render3d_st, c, params, n_params, bufs, groups, 1, 1) }
 }
+# a compute program that also samples n_tex textures (bound after its buffers), and its 2-D dispatch
+function gpu_compute_tex(render3d_st: mut Render3dState, name: string, n_bufs: int, n_tex: int) -> int {
+  if render3d_st.gpu_kind != GPU_VK { return 0 }
+  return gvk_compute_new_tex(render3d_st, name, n_bufs, n_tex)
+}
+function gpu_dispatch_tex(render3d_st: mut Render3dState, c: int, params: pointer, n_params: int, bufs: words, texs: words, gx: int, gy: int) -> void {
+  if render3d_st.gpu_kind == GPU_VK and c > 0 { gvk_dispatch_tex(render3d_st, c, params, n_params, bufs, texs, gx, gy, 1) }
+}
 # a buffer a compute pass writes (never reallocated under a draw that reads it)
 function gpu_buffer_gpu_owned(render3d_st: mut Render3dState, buf: int) -> void { if render3d_st.gpu_kind == GPU_VK { gvk_buf_gpu_owned(render3d_st, buf) } }
 # the host-visible contents of a buffer, for a readback after gpu_finish; null on OpenGL