perf(render): batch model rendering with instancing

Replaces per-entity draw calls with instanced rendering for both main and shadow passes. Each model's node hierarchy is flattened once and instance matrices are updated per frame, reducing draw calls and CPU overhead.

assets/shaders/depth_model_vert copy.glsl

@@ -0,0 +1,14 @@
+#version 460
+
+layout (location = 0) in vec3 pos;
+layout (location = 1) in vec2 texCoord;
+
+uniform mat4 lightSpaceMatrix;
+uniform mat4 modelMatrix;
+
+out vec2 tc;
+
+void main() {
+    tc = texCoord;
+    gl_Position = lightSpaceMatrix * modelMatrix * vec4(pos, 1.0);
+}