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>

crates/render/src/lib.rs

@@ -39,6 +39,10 @@ pub trait FootageSource: Send + Sync {
     fn audio(&self, _item: ItemId, _footage: &Footage, _t: Tick, _frames: usize, _rate: u32) -> Option<Vec<f32>> {
         None
     }
+    /// The parsed 3D model of a [`effectcraft_project::FootageKind::Model`] item (Advanced 3D).
+    fn model(&self, _item: ItemId, _footage: &Footage) -> Option<Arc<effectcraft_model::Model>> {
+        None
+    }
     /// Set the decoded-frame cache budget in bytes (Settings ▸ Memory & CPU); sources without
     /// a cache ignore it.
     fn set_cache_budget(&self, _bytes: usize) {}