feat(view): ③ rebar view at 50% for members, ground and blinding; deeper rebar colour

On ③ the members, the ground and the blinding concrete are drawn at half opacity without depth writes, so the bars show
through. Outside ③ each part returns to its own opacity. Rebar colours are shaded to 0.8 and slightly glossier so they
read through the members. Approved 2026-10-23.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

src/view/scene.ts

@@ -87,6 +87,7 @@ export function buildPierScene(m: PierModel): PierScene {
       side: THREE.DoubleSide,
       clippingPlanes,
     });
+    material.userData.baseOpacity = opacity;
     parts[part].add(new THREE.Mesh(geometry, material));
   };
 
@@ -146,6 +147,24 @@ export function buildPierScene(m: PierModel): PierScene {
   return { root, parts, size };
 }
 
+/** ③ rebar view: members and the ground at half opacity so the bars show through (승인 2026-10-23). */
+const REBAR_VIEW_ALPHA = 0.5;
+
+/** Turns the ③ rebar view on or off. Off restores each part's own opacity. Depth writes stop while on, so bars behind a member stay visible. */
+export function setRebarView(scene: PierScene, on: boolean): void {
+  for (const group of Object.values(scene.parts)) {
+    group.traverse((o) => {
+      if (!(o instanceof THREE.Mesh)) return;
+      const mat = o.material as THREE.MeshStandardMaterial;
+      const base = mat.userData.baseOpacity as number;
+      mat.opacity = on ? REBAR_VIEW_ALPHA : base;
+      mat.transparent = on || base < 1;
+      mat.depthWrite = !on;
+      mat.needsUpdate = true;
+    });
+  }
+}
+
 export type ViewKind = 'iso' | 'front' | 'side' | 'top';
 
 /** Camera position for one of the four view presets, looking at the centre of the model. */