feat(webgl): Enable all 28 scenarios for WebGL test harness

- Update wasm/Makefile to compile ALL scenario files using wildcard
- Add conditional compilation to scenario_bitmap.cpp (#ifdef __EMSCRIPTEN__)
  to handle OpenGL-specific shader workaround code for native only
- Change canvas dimensions to 1600x1200 to match native baseline (2x Retina)
- Set white background color to match native screenshots
- Update gal-webgl.spec.ts to hide UI overlay before taking screenshots
- Improve test-gal-regression.sh with better ImageMagick comparison

All 28 scenarios now compile and run on both native and WebGL backends.
Native vs baseline: PASSED (28/28 matching)
WebGL rendering: In progress (primitives not yet visible)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/e2e/gal-webgl.spec.ts

@@ -138,10 +138,16 @@ test.describe('GAL WebGL Regression Tests', () => {
 
       await expect(canvas).toBeVisible({ timeout: 5000 });
 
+      // Hide controls overlay before screenshot (it sits on top of canvas)
+      await page.locator('#controls-overlay').evaluate(el => el.style.visibility = 'hidden');
+
       // Screenshot the canvas (matching native 800x600 output)
       const screenshotPath = path.join(OUTPUT_DIR, `gal-${scenarioName}.png`);
       await canvas.screenshot({ path: screenshotPath });
 
+      // Restore overlay for manual debugging
+      await page.locator('#controls-overlay').evaluate(el => el.style.visibility = 'visible');
+
       // Print console logs for first scenario (debugging)
       if (scenarioIndex === 0) {
         console.log('\n=== Console logs ===');
@@ -174,11 +180,17 @@ test.describe('GAL WebGL Regression Tests', () => {
       // Wait for rendering
       await page.waitForTimeout(50);
 
+      // Hide controls overlay before screenshot
+      await page.locator('#controls-overlay').evaluate(el => el.style.visibility = 'hidden');
+
       // Screenshot the canvas
       const canvas = await page.locator('#canvas');
       const screenshotPath = path.join(OUTPUT_DIR, `gal-${scenarioName}.png`);
       await canvas.screenshot({ path: screenshotPath });
 
+      // Restore overlay
+      await page.locator('#controls-overlay').evaluate(el => el.style.visibility = 'visible');
+
       console.log(`[${i + 1}/28] ${scenarioName}`);
     }