wip: checkpoint mid-refactor of asset loading before submesh work

Moves mesh/material loaders under src/assets/loaders, renames Mesh to
MeshHandle, and begins scaffolding the material loader. Known-broken
build state (MeshRegistry::from_scene call site and asset_manager
mesh registration are out of sync) — committed as a rollback point
before implementing submeshes.

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

src/assets/asset_manager.rs

@@ -0,0 +1,57 @@
+use std::path::Path;
+use std::sync::Arc;
+use anyhow::Result;
+use vulkano::memory::allocator::MemoryAllocator;
+use crate::assets::loaders::material::mat_loader::MaterialLoader;
+use crate::assets::loaders::mesh_loader::MeshLoader;
+use crate::common::material::MaterialRegistry;
+use crate::common::mesh::{MeshHandle, MeshRegistry};
+
+/// A manager that controls all assets used for display. It handles loading all meshes, materials,
+/// and textures for now.
+pub struct AssetManager {
+    /// The asset manager's [`MeshRegistry`].
+    mesh_registry: MeshRegistry,
+    /// The asset manager's [`MaterialRegistry`].
+    material_registry: MaterialRegistry,
+
+    // Loaders
+    mesh_loader: MeshLoader,
+    material_loader: MaterialLoader,
+}
+
+impl AssetManager {
+    /// Creates a new `AssetManager` instance.
+    pub fn new(allocator: Arc<dyn MemoryAllocator>) -> Self {
+        let mesh_registry = MeshRegistry::new(allocator.clone());
+        let material_registry = MaterialRegistry::new();
+        Self {
+            mesh_registry,
+            material_registry,
+            mesh_loader: MeshLoader::new(),
+            material_loader: MaterialLoader::new(),
+        }
+    }
+
+    /// Loads a full model from its file path. This loads the model's mesh, textures, and material
+    /// definitions, registers them with the `AssetLoader`'s registries, and prepares them for
+    /// display. Returns a blank `Result` indicating success or failure.
+    ///
+    /// This method will attempt to auto-generate IDs for materials and textures if they do not have
+    /// any preconfigured, using the ID of the mesh and the type of resource being stored.
+    pub fn load_model<P: AsRef<Path>>(&mut self, mesh_id: String, path: P) -> Result<()> {
+        // Load and register all materials first - submeshes will reference these materials.
+        let materials = self.material_loader.load_materials(&path)?;
+
+        // Load and register the mesh using the materials
+        let mesh = self.mesh_loader.load_mesh(&path)?;
+        self.mesh_registry.register(mesh_id, mesh);
+
+        Ok(())
+    }
+
+    /// Loads all information from a scene, returning an empty `Result` indicating success or failure.
+    pub fn load_scene<P: AsRef<Path>>(&self, path: P) -> Result<()> {
+        Ok(())
+    }
+}