feat: Stage 2C CAD assembly cleanup, full-height canvas, physics parity

Remove default Measurement overlay and procedural motor duplicate; force
opaque CAD materials; modular extension pitch from CAD; shared runtime/
headless physics config hash; layout e2e and Stage 2C artifacts.

Co-authored-by: Cursor <cursoragent@cursor.com>

e2e/stage2c-layout.spec.ts

@@ -0,0 +1,49 @@
+import { test, expect } from '@playwright/test';
+
+test.setTimeout(90_000);
+
+test.describe('Stage 2C layout + measurement panel', () => {
+  test('default / has no Measurement panel and canvas fills viewport height', async ({ page }) => {
+    await page.goto('/?playback=paused');
+    await expect(page.locator('canvas')).toBeVisible({ timeout: 60_000 });
+    await page.waitForFunction(() => !document.body.innerText.includes('Loading 3D'), {
+      timeout: 60_000,
+    });
+    await page.waitForTimeout(500);
+
+    await expect(page.locator('.cv-overlay')).toHaveCount(0);
+    await expect(page.getByTestId('measurement-compact')).toHaveCount(0);
+
+    const metrics = await page.evaluate(() => {
+      const vh = window.innerHeight;
+      const canvas = document.querySelector('canvas');
+      const svg = document.querySelector('[data-testid="main-svg-fallback"]');
+      const scene = canvas ?? svg;
+      if (!scene) return { ok: false as const, reason: 'no scene' };
+      const r = scene.getBoundingClientRect();
+      const heightRatio = r.height / vh;
+      const emptyBelow = Math.max(0, vh - (r.top + r.height));
+      const emptyRatio = emptyBelow / vh;
+      return {
+        ok: true as const,
+        heightRatio,
+        emptyRatio,
+        sceneTop: r.top,
+        sceneHeight: r.height,
+        vh,
+      };
+    });
+
+    expect(metrics.ok).toBe(true);
+    if (!metrics.ok) return;
+    expect(metrics.heightRatio).toBeGreaterThanOrEqual(0.85);
+    expect(metrics.emptyRatio).toBeLessThanOrEqual(0.2);
+  });
+
+  test('full Measurement panel only with debug=1&measurement=full', async ({ page }) => {
+    await page.setViewportSize({ width: 1280, height: 800 });
+    await page.goto('/?debug=1&playback=paused');
+    await expect(page.locator('canvas')).toBeVisible({ timeout: 60_000 });
+    await expect(page.locator('.cv-overlay:not(.cv-overlay-compact)')).toHaveCount(0);
+  });
+});