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>

packages/ludic.render3d/env.ludic

@@ -99,6 +99,7 @@ export state Render3dState {
   gltf_comps: int = 0
   gltf_tex_paths: []pointer = null
   gltf_tex_ids: words = null
+  gltf_tex_refs: words = null        # how many primitives use each cached texture (model_release)
   gltf_tex_n: int = 0
   gltf_white: int = 0
   gltf_flat: int = 0