feat: add submesh support for mesh loading and rendering

Meshes now track per-primitive submesh ranges (index range + source
material index) instead of one flat index range each. glTF meshes get
one submesh per primitive; OBJ meshes get a single default submesh
until .mtl loading exists. MeshRegistry rewrites these into offsets
into its shared index buffer, and the renderer issues one draw call
per submesh so each can later be bound to its own material.

Also fixes app.rs to match the current MeshRegistry API (load_scene
instead of the removed from_scene, and Option-wrapped buffers).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01986QNHM1cUsnRskKvBbbuo

src/common/mesh.rs

@@ -19,10 +19,29 @@ pub struct MeshHandle {
     pub vertex_count: usize,
     pub index_loc: usize,
     pub index_count: usize,
+    /// The submeshes that make up this mesh, in the order they should be drawn. Each one covers
+    /// a contiguous range of the registry's shared index buffer and (once material loading is
+    /// wired up) should be drawn with its own material bound.
+    pub submeshes: Vec<SubmeshHandle>,
 }
 
 impl MeshHandle {}
 
+/// A submesh's location within a [`MeshRegistry`]'s shared index buffer, along with a reference
+/// to the material it was assigned in its source file.
+///
+/// Unlike [`crate::assets::loaders::mesh_loader::Submesh`], `index_loc` here is an absolute
+/// offset into the registry's shared index buffer rather than an offset local to one loaded file.
+#[derive(Debug, Clone)]
+pub struct SubmeshHandle {
+    /// The material index this submesh was assigned in its source file, if any. This is not yet
+    /// resolved to a [`crate::common::material::Material`] - that mapping is the caller's
+    /// responsibility until material loading is wired into mesh registration.
+    pub material_index: Option<usize>,
+    pub index_loc: usize,
+    pub index_count: usize,
+}
+
 type VertexBuffer = Arc<Subbuffer<[Vertex3]>>;
 type IndexBuffer = Arc<Subbuffer<[u32]>>;
 
@@ -102,9 +121,19 @@ impl MeshRegistry {
             let obj_num_idx = obj.indices.len();
             buf_indices.extend(obj.indices);
 
+            let submeshes = obj
+                .submeshes
+                .into_iter()
+                .map(|submesh| SubmeshHandle {
+                    material_index: submesh.material_index,
+                    index_loc: index_start + submesh.index_start,
+                    index_count: submesh.index_count,
+                })
+                .collect::<Vec<_>>();
+
             println!(
-                "Loaded mesh \"{}\" with {} vertices and {} indices",
-                id, obj_num_verts, obj_num_idx
+                "Loaded mesh \"{}\" with {} vertices, {} indices, and {} submesh(es)",
+                id, obj_num_verts, obj_num_idx, submeshes.len()
             );
 
             // TODO Create bounding boxes for each mesh and associate them with the struct.
@@ -116,6 +145,7 @@ impl MeshRegistry {
                     vertex_count: obj_num_verts,
                     index_loc: index_start,
                     index_count: obj_num_idx,
+                    submeshes,
                 },
             );
         });