받침 중심 정렬 + 부재별 색상 구분

bearing.rs: X 중심, Y 하방향 (-h to 0) 기하학 수정
bridge_scene.rs: 받침 X 오프셋 제거 (girder 정렬)
Mesh: colors 필드 추가 + recolor() 메서드
sweep.rs / occt.rs: 기본 콘크리트 색 자동 채움
bridge_scene: 부재별 색상 (거더/슬래브/받침/교대)
shader.wgsl: base_color 입력 → 조명 계산에 적용

선택(Selection) 기능은 계획대로 별도 Sprint에 구현.

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

cimery/crates/viewer/src/lib.rs

@@ -33,18 +33,20 @@ use glam;
 
 // ─── Vertex ───────────────────────────────────────────────────────────────────
 
-/// Per-vertex data sent to GPU: 3D position + surface normal.
+/// Per-vertex data sent to GPU: position + normal + base color.
 #[repr(C)]
 #[derive(Copy, Clone, Debug, Pod, Zeroable)]
 struct Vertex {
-    position: [f32; 3],
-    normal:   [f32; 3],
+    position:   [f32; 3],
+    normal:     [f32; 3],
+    base_color: [f32; 3],  // material base colour (modulated by lighting)
 }
 
 impl Vertex {
-    const ATTRIBS: [wgpu::VertexAttribute; 2] = wgpu::vertex_attr_array![
+    const ATTRIBS: [wgpu::VertexAttribute; 3] = wgpu::vertex_attr_array![
         0 => Float32x3,  // position
         1 => Float32x3,  // normal
+        2 => Float32x3,  // base_color
     ];
 
     fn desc() -> wgpu::VertexBufferLayout<'static> {
@@ -151,8 +153,10 @@ impl RenderState {
         let mesh = bridge_scene::build_bridge_scene(&PureRustKernel)
             .expect("PureRustKernel bridge scene");
 
-        let verts: Vec<Vertex> = mesh.vertices.iter().zip(mesh.normals.iter())
-            .map(|(p, n)| Vertex { position: *p, normal: *n })
+        let verts: Vec<Vertex> = mesh.vertices.iter()
+            .zip(mesh.normals.iter())
+            .zip(mesh.colors.iter())
+            .map(|((p, n), c)| Vertex { position: *p, normal: *n, base_color: *c })
             .collect();
 
         let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {