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>

.gitignore

@@ -49,6 +49,8 @@ wxwidgets-clean/
 /tests/apps/kicad/*.wasm
 /tests/apps/kicad/*.tar.gz
 !tests/apps/kicad/pcbnew.html
+/tests/apps/gal-webgl/*.js
+/tests/apps/gal-webgl/*.wasm
 /temp/
 *.log
 *.tmp