fix(renderer): merge IBL into group(3) to stay within max_bind_groups limit of 4

wgpu's default max_bind_groups is 4 (groups 0-3), but the PBR shader was
using group(4) for BRDF LUT bindings. This merges IBL bindings into the
shadow bind group (group 3) at binding slots 3-4, removes the standalone
IBL bind group layout/creation, and updates all examples accordingly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

examples/multi_light_demo/src/main.rs

@@ -40,7 +40,6 @@ struct AppState {
     shadow_bind_group: wgpu::BindGroup,
     _shadow_map: ShadowMap,
     _ibl: IblResources,
-    ibl_bind_group: wgpu::BindGroup,
     input: InputState,
     timer: GameTimer,
     cam_aligned_size: u32,
@@ -190,8 +189,6 @@ impl ApplicationHandler for MultiLightApp {
 
         // IBL resources
         let ibl = IblResources::new(&gpu.device, &gpu.queue);
-        let ibl_layout = IblResources::bind_group_layout(&gpu.device);
-        let ibl_bind_group = ibl.create_bind_group(&gpu.device, &ibl_layout);
 
         // Material bind group
         let material_bind_group = gpu.device.create_bind_group(&wgpu::BindGroupDescriptor {
@@ -227,6 +224,8 @@ impl ApplicationHandler for MultiLightApp {
             &gpu.device,
             &shadow_layout,
             &shadow_uniform_buffer,
+            &ibl.brdf_lut_view,
+            &ibl.brdf_lut_sampler,
         );
 
         // PBR pipeline
@@ -237,7 +236,6 @@ impl ApplicationHandler for MultiLightApp {
             &pbr_tex_layout,
             &mat_layout,
             &shadow_layout,
-            &ibl_layout,
         );
 
         self.state = Some(AppState {
@@ -259,7 +257,6 @@ impl ApplicationHandler for MultiLightApp {
             shadow_bind_group,
             _shadow_map: shadow_map,
             _ibl: ibl,
-            ibl_bind_group,
             input: InputState::new(),
             timer: GameTimer::new(60),
             cam_aligned_size,
@@ -545,7 +542,6 @@ impl ApplicationHandler for MultiLightApp {
                     render_pass.set_pipeline(&state.pipeline);
                     render_pass.set_bind_group(1, &state.pbr_texture_bind_group, &[]);
                     render_pass.set_bind_group(3, &state.shadow_bind_group, &[]);
-                    render_pass.set_bind_group(4, &state.ibl_bind_group, &[]);
 
                     // Draw 5 spheres (objects 0..4)
                     render_pass.set_vertex_buffer(0, state.sphere_mesh.vertex_buffer.slice(..));