M7.4: model import

New crate effectcraft-model (L2): glTF 2.0 (.gltf/.glb, data URIs, external buffers and
images, PBR metallic-roughness with base/MR/normal/occlusion/emissive textures, node
hierarchy, skins, step/linear/cubic animations) and OBJ/MTL import, parametric primitives,
extruded/bevelled outline meshes and polygon triangulation with holes.

Project: FootageKind::Model, LayerSource::Model/Primitive, Layer::environment, Geometry and
Material Options builders, Environment light Source. Media probes and loads models
(FootageSource::model). Engine: layer.newModel, layer.new3dPrimitive/newPrimitive,
comp.renderer, layer.environment, material.set/reset/duplicateAssign/revealSource (stubs
replaced); File > Import and layer.addItem pick up models. Hand-written fixtures with
attribution sidecars.

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

xtask/src/main.rs

@@ -31,6 +31,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("text", 2),
     ("effects", 2),
     ("track", 2),
+    ("model", 2),
     ("render", 3),
     ("media", 3),
     ("expr", 3),