feat(render3d): load steps for a loading screen, four quality switches, and the ring's uniform usage

- r3d_open opens the window with nothing baked; r3d_load_count / r3d_load_step run the rest (sky
  and light, terrain, shadow and screen targets, cover and actors, grass) so a game can present a
  loading frame between them. r3d_init is the same calls in a row.
- sky_set_quality(width): the prefiltered sky light at 256 / 512 / 1024, baked again.
- post_set_msaa(samples): multisampling on OpenGL, remade in place (post_msaa_live is false on
  Vulkan); post_free frees the multisampled framebuffer too.
- STREAM_BUDGET_US is a variable; r3d_fog_scale multiplies the fog the day sets.
- Vulkan buffers carry UNIFORM_BUFFER usage: the frame's ring is bound as uniform buffers and was
  created without it (VUID-VkWriteDescriptorSet-descriptorType-00330, found on MoltenVK).

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/daylight.ludic

@@ -143,7 +143,7 @@ function daylight_set(hours: int) -> void {
   let dim = f_sub(F_ONE, f_mul(fl(0.55), oc))
   for i in 0 .. 3 { day_ibl[i] = f_mul(f_lerp(day_ibl[i], grey, f_mul(fl(0.7), oc)), dim) }
   if day_fog_base == 0 { day_fog_base = r3d_fog_density }
-  r3d_fog_density = f_mul(day_fog_base, day_fog_mul)
+  r3d_fog_density = f_mul(f_mul(day_fog_base, day_fog_mul), r3d_fog_scale)
   # exposure: auto-exposure must not turn the night into day
   # the ceiling has to move with the moon or auto-exposure eats the difference between a
   # full-moon night and a new-moon one