fix(view): ③ 철근배근도 선택한 구조물만 3D에 표시

- 교각을 선택하면 교대를 숨김(②와 같은 규칙). 교대 선택 시 교각 숨김은 기존대로
- ① ↔ ③ 전환 때도 장면을 다시 만들어 표시 규칙이 바로 반영되게 함

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

src/main.ts

@@ -2559,8 +2559,9 @@ function setStage(stage: Stage): void {
   const previous = state.stage;
   state.stage = stage;
   // Switching 전체 보기 on or off in ① needs the scene rebuilt for the new layout.
-  // Entering or leaving ② changes which objects are drawn (visibility.ts), so the scene is rebuilt then.
-  if ((allPiers !== null) !== multiSceneOn() || stage === 'plan' || previous === 'plan') setupScene();
+  // Entering or leaving ② or ③ changes which objects are drawn (visibility.ts), so the scene is rebuilt then.
+  const selectionOnly = (s: Stage) => s === 'plan' || s === 'rebar';
+  if ((allPiers !== null) !== multiSceneOn() || selectionOnly(stage) !== selectionOnly(previous)) setupScene();
   if (state.scene) setRebarView(state.scene, stage === 'rebar');
   for (const g of abutGroups) setRebarViewOn(g, stage === 'rebar');
   $<HTMLElement>('view-shape').hidden = stage !== 'shape';