feat(render3d): 23.4 - model_release, shared textures counted, and scatter layers sized to what they hold

- model_release(model) frees each primitive's mesh, and each texture once no other model uses it:
  the glTF texture cache counts the primitives using each texture (a path loaded again, and a LOD
  chain borrowing its LOD0's material, each take a reference); the last one frees the texture and
  forgets it along with any remembered material naming it, so a later load is a fresh one
- a scatter layer's instance and sort arrays start at 256 and double as layer_add or a stream
  fills them, to the layer's cap, instead of the whole cap up front (0.4 GB in Maroon Lake)
- gvk_tex_read's copy struct comes from the scratch ring
- examples/rendering/release.ludic holds it (RELEASE OK on Vulkan and OpenGL), in the suite

smooth renders pixel-identical before and after (max |d| 0). ludic-dev test 306/306.

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

changes/model-release.md

@@ -0,0 +1,10 @@
+bump: minor
+type: feature
+**A loaded model can be let go of: `model_release(model)`.** Its meshes are freed at once, and
+each texture once no other model uses it - the glTF loader shares textures between models (one
+path loaded once, a LOD chain borrowing its LOD0's material), so every cached texture now counts
+the primitives using it, and the last one frees it and forgets it, with any remembered material
+that named it. A game can unload a species' kit or a map's models when nothing of them is left,
+and loading one again afterwards is a fresh load. A scatter layer takes room for what it holds
+instead of its whole capacity up front (it doubles as it fills, to its cap), which in Maroon Lake
+was 0.4 GB of never-filled instance arrays. `examples/rendering/release.ludic` is the check.