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/stream.ludic

@@ -143,7 +143,7 @@ function stream_band(s: Stream, d: int) -> int {
 # by band and kind. With a real microsecond clock the budget can just be the thing we
 # actually care about — how long this frame is allowed to spend growing ground cover.
 # Overshoot is bounded by one chunk, so keep chunks small on the dense near streams.
-const STREAM_BUDGET_US: int = 2500    # microseconds of generation per frame
+var STREAM_BUDGET_US: int = 2500      # microseconds of generation per frame; a setting may move it
 var stream_deadline: long = 0
 const STREAM_BUDGET: int = 8000       # kept for the work counter only
 # The worst frame is now bounded by one chunk, not by the budget: stream_fill emits a