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>

changes/quality-switches.md

@@ -0,0 +1,11 @@
+bump: minor
+type: feat
+**More of the renderer's quality is a switch a game can offer.**
+
+- `sky_set_quality(width)`: the prefiltered sky light at 256, 512 or 1024 wide (half as tall),
+  baked again at once.
+- `post_set_msaa(samples)`: 1, 2 or 4 samples for the scene on OpenGL, remaking the screen targets in
+  place (`post_msaa_live()` is false on Vulkan, which has no sample counts yet); `post_free` now
+  frees the multisampled framebuffer too.
+- `STREAM_BUDGET_US` is a variable: the microseconds a frame may spend generating streamed cover.
+- `r3d_fog_scale`: a multiplier over the fog density the daylight and the weather set.

changes/r3d-load-steps.md

@@ -0,0 +1,7 @@
+bump: minor
+type: feat
+**A loading screen can show the renderer's start-up as it happens.** `r3d_open(w, h, title)` opens
+the window and the graphics backend with nothing baked, and `r3d_load_count()` / `r3d_load_step(i)`
+run the rest - the sky and its light, the terrain, the shadow and screen targets, the scattered cover
+and actors, the grass - one step at a time, so a game can draw and present a frame between them.
+`r3d_init` is those same calls in a row and is unchanged for everything that uses it.