feat(webgl): Add WebGL GAL test infrastructure (Phase 1)

Add complete test infrastructure for WebGL GAL visual regression testing:

- scripts/test-gal-regression.sh: Master script that builds both backends,
  runs tests, and performs two-level comparison (native vs baseline,
  webgl vs native)
- scripts/build-gal-webgl-test.sh: WASM build using Makefile with em++
- tests/gal-regression/wasm/: WebGL test harness (stub WEBGL_GAL)
- tests/e2e/gal-webgl.spec.ts: Playwright test for screenshot capture

Fix Homebrew Emscripten environment in scripts/common/env.sh:
- Set EMSDK_PYTHON for Python 3.10+ (em++ reads this, not $PYTHON)
- Add bundled LLVM to PATH (Emscripten needs its clang with WASM backend)

Verified: Native vs Baseline passes (28/28), WebGL generates blank
screenshots as expected (WEBGL_GAL implementation is Phase 2).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/e2e/gal-webgl.spec.ts

@@ -0,0 +1,142 @@
+/**
+ * GAL WebGL Regression Test
+ *
+ * Runs all 28 GAL test scenarios in WebGL and captures screenshots
+ * for comparison against native OpenGL rendering.
+ */
+
+import { test, expect } from './utils/fixtures';
+import * as path from 'path';
+import * as fs from 'fs';
+
+// Scenario names (must match native test)
+const SCENARIO_NAMES = [
+  'basic-lines',      // 0
+  'line-widths',      // 1
+  'circles',          // 2
+  'arcs',             // 3
+  'rectangles',       // 4
+  'polygons',         // 5
+  'alpha-blending',   // 6
+  'transforms',       // 7
+  'grid-cursor',      // 8
+  'segments',         // 9
+  'complex-scene',    // 10
+  'bezier-curves',    // 11
+  'arc-segments',     // 12
+  'segment-chain',    // 13
+  'group-caching',    // 14
+  'polylines-multi',  // 15
+  'hole-walls',       // 16
+  'grid-native',      // 17
+  'cursor-native',    // 18
+  'render-targets',   // 19
+  'screen-transform', // 20
+  'clear-colors',     // 21
+  'depth-testing',    // 22
+  'negative-mode',    // 23
+  'text-attrs',       // 24
+  'glyphs',           // 25
+  'bitmap',           // 26
+  'transform-api'     // 27
+];
+
+// Output directory for WebGL screenshots
+const OUTPUT_DIR = path.join(__dirname, '../gal-regression/output/webgl');
+
+test.describe('GAL WebGL Regression Tests', () => {
+  test.beforeAll(async () => {
+    // Ensure output directory exists
+    if (!fs.existsSync(OUTPUT_DIR)) {
+      fs.mkdirSync(OUTPUT_DIR, { recursive: true });
+    }
+  });
+
+  test('Load GAL WebGL test module', async ({ page, testLogger }) => {
+    await page.goto('/gal-webgl/gal_webgl_test.html');
+
+    // Wait for the custom event indicating module is ready
+    await page.waitForFunction(() => {
+      return (window as any).galTest !== undefined;
+    }, { timeout: 60000 });
+
+    // Verify module loaded
+    const totalScenarios = await page.evaluate(() => {
+      return (window as any).galTest.getTotalScenarios();
+    });
+
+    expect(totalScenarios).toBe(28);
+
+    await page.screenshot({
+      path: path.join(OUTPUT_DIR, 'gal-module-loaded.png'),
+      fullPage: true
+    });
+
+    console.log(`GAL WebGL test module loaded with ${totalScenarios} scenarios`);
+  });
+
+  // Generate a test for each scenario
+  for (let i = 0; i < SCENARIO_NAMES.length; i++) {
+    const scenarioName = SCENARIO_NAMES[i];
+    const scenarioIndex = i;
+
+    test(`Scenario ${scenarioIndex}: ${scenarioName}`, async ({ page, testLogger }) => {
+      await page.goto('/gal-webgl/gal_webgl_test.html');
+
+      // Wait for module to be ready
+      await page.waitForFunction(() => {
+        return (window as any).galTest !== undefined;
+      }, { timeout: 60000 });
+
+      // Run the scenario
+      await page.evaluate((index) => {
+        (window as any).galTest.runScenario(index);
+      }, scenarioIndex);
+
+      // Wait for rendering to complete
+      await page.waitForTimeout(100);
+
+      // Get the canvas element and take a screenshot of just the canvas
+      const canvas = await page.locator('#canvas');
+      await expect(canvas).toBeVisible();
+
+      // Screenshot the canvas (matching native 800x600 output)
+      const screenshotPath = path.join(OUTPUT_DIR, `gal-${scenarioName}.png`);
+      await canvas.screenshot({ path: screenshotPath });
+
+      console.log(`Saved: ${screenshotPath}`);
+    });
+  }
+
+  test('Run all scenarios sequentially', async ({ page, testLogger }) => {
+    await page.goto('/gal-webgl/gal_webgl_test.html');
+
+    // Wait for module to be ready
+    await page.waitForFunction(() => {
+      return (window as any).galTest !== undefined;
+    }, { timeout: 60000 });
+
+    console.log('Running all 28 scenarios...');
+
+    for (let i = 0; i < SCENARIO_NAMES.length; i++) {
+      const scenarioName = SCENARIO_NAMES[i];
+
+      // Run scenario
+      await page.evaluate((index) => {
+        (window as any).galTest.runScenario(index);
+      }, i);
+
+      // Wait for rendering
+      await page.waitForTimeout(50);
+
+      // Screenshot the canvas
+      const canvas = await page.locator('#canvas');
+      const screenshotPath = path.join(OUTPUT_DIR, `gal-${scenarioName}.png`);
+      await canvas.screenshot({ path: screenshotPath });
+
+      console.log(`[${i + 1}/28] ${scenarioName}`);
+    }
+
+    console.log('All scenarios completed');
+  });
+});