perf(render3d): the sky light baked once at start, the terrain load in four steps, actor cull bounds by height

- sky_start_yaw: a game that turns the sky at boot sets it before r3d_init and the image-based light
  is baked once at that yaw; sky_set_yaw to a yaw already baked bakes nothing.
- terrain_init_step: the height field and normals, the sun shadow, the materials, the patches and
  programs as separate steps; r3d_load_count is 8, so a loading bar moves through the terrain (half
  the start-up) instead of jumping over it. terrain_init runs the four in a row as before.
- The per-cascade actor cull centres its sphere at half the model's height and sizes it by the
  larger of height and radius: centred at the radius, it dropped a head at a cascade's edge (11 px
  in town, found by the drawstats comparison). Lake was byte-identical before and after.

OpenGL frames at the five viewpoints unchanged; the game's 59 self-tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/ludic.render3d/terrain.ludic

@@ -387,9 +387,21 @@ function terrain_load_textures() -> void {
 }
 
 function terrain_init() -> void {
-  terrain_generate()
-  terrain_bake_shadow()
-  terrain_load_textures()
+  for i in 0 .. TERRAIN_INIT_STEPS { terrain_init_step(i) }
+}
+
+# The terrain's start-up as steps a loading screen can show between, in terrain_init's order:
+# 0 the height field and its normals (and the read-back for placement), 1 the sun's height-field
+# shadow, 2 the ground's materials, 3 the patch tree and the terrain programs.
+const TERRAIN_INIT_STEPS: int = 4
+function terrain_init_step(i: int) -> void {
+  if i == 0 { terrain_generate();  return }
+  if i == 1 { terrain_bake_shadow();  return }
+  if i == 2 { terrain_load_textures();  return }
+  terrain_init_finish()
+}
+
+function terrain_init_finish() -> void {
   cdlod_init()
   ter_wire = Os.has_env("R3D_WIRE")
   ter_force_far = Os.has_env("R3D_TFARONLY")