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>

tests/rebar3d.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it } from 'vitest';
 import * as THREE from 'three';
 import { buildPier } from '../src/core/geometry';
 import { buildRebar3D } from '../src/view/rebar3d';
+import { buildPierScene, setRebarView } from '../src/view/scene';
 import { DEFAULT_MATERIAL } from '../src/core/rebar/common';
 import { loadP1 } from './helpers';
 
@@ -119,3 +120,27 @@ describe('3D 철근 모델은 ③ 공통 설정 fck를 따른다', () => {
     expect(high).toBeLessThan(base - 0.05);
   });
 });
+
+describe('③ 부재 반투명 (승인 2026-10-23)', () => {
+  const model = buildPier(loadP1());
+  const opacities = (scene: ReturnType<typeof buildPierScene>) => {
+    const out: Record<string, number[]> = {};
+    for (const [key, group] of Object.entries(scene.parts)) {
+      group.traverse((o) => {
+        if (o instanceof THREE.Mesh) (out[key] ??= []).push((o.material as THREE.MeshStandardMaterial).opacity);
+      });
+    }
+    return out;
+  };
+
+  it('members and the ground go to 50% on ③ and back to their own opacity off ③', () => {
+    const scene = buildPierScene(model);
+    const before = opacities(scene);
+    setRebarView(scene, true);
+    for (const key of ['coping', 'columns', 'footings', 'piles', 'blinding', 'blocks', 'pads', 'ground']) {
+      expect(opacities(scene)[key].every((o) => o === 0.5), key).toBe(true);
+    }
+    setRebarView(scene, false);
+    expect(opacities(scene)).toEqual(before);
+  });
+});