fix(view): move the ③ colour control into the 3D viewer window

The colour basis control and legend sit below the 3D canvas and show only on ③, so they belong to the model window.

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

src/main.ts

@@ -709,6 +709,7 @@ function placeViewer(stage: 'shape' | 'plan' | 'rebar'): void {
   const slot = stage === 'plan' ? $<HTMLElement>('slot-plan') : stage === 'rebar' ? $<HTMLElement>('slot-rebar') : null;
   if (slot) slot.appendChild(block);
   else if (viewerHome.parent) viewerHome.parent.insertBefore(block, viewerHome.next);
+  renderRebarColor();
   resize();
 }
 
@@ -771,6 +772,8 @@ const REBAR_MEMBERS: { id: RebarRow['member']; label: string }[] = [
 
 /** 3D colour basis buttons on ③ and their legend (spec §6.7). */
 function renderRebarColor(): void {
+  // The colour control lives in the 3D window and is shown only on ③.
+  $<HTMLDivElement>('rebar-color').hidden = state.stage !== 'rebar';
   for (const b of document.querySelectorAll<HTMLButtonElement>('#rebar-color button')) {
     b.setAttribute('aria-pressed', String(b.dataset.color === state.rebarColor));
   }