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>

scripts/build-gal-webgl-test.sh

@@ -0,0 +1,77 @@
+#!/bin/bash
+#
+# Build script for the WebGL GAL test harness (WASM)
+#
+# This builds a WASM module that renders GAL test scenarios using WebGL,
+# allowing comparison against native OpenGL rendering.
+#
+# Uses a Makefile with direct em++ calls (like build-wasm-test.sh)
+# to avoid emcmake Python 3.10+ requirement.
+#
+
+# Redirect all output to a log file (re-execs script with redirection)
+source "$(dirname "$0")/common/logging.sh"
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+# Source common environment (sets up Python 3.10+ for Emscripten)
+QUIET=1 source "$SCRIPT_DIR/common/env.sh"
+
+TEST_DIR="$PROJECT_ROOT/tests/gal-regression/wasm"
+OUTPUT_DIR="$PROJECT_ROOT/tests/apps/gal-webgl"
+
+echo "Building GAL WebGL Test..."
+echo "  Test dir: $TEST_DIR"
+echo "  Output dir: $OUTPUT_DIR"
+echo "  EMSDK_PYTHON: ${EMSDK_PYTHON:-NOT SET}"
+echo "  clang: $(which clang)"
+
+# Verify em++ is available
+if ! command -v em++ &> /dev/null; then
+    echo "ERROR: em++ not found. Please install via: brew install emscripten"
+    exit 1
+fi
+
+echo "  Emscripten: $(em++ --version 2>&1 | head -1)"
+
+# Parse arguments
+DEBUG_BUILD=0
+CLEAN_BUILD=0
+
+for arg in "$@"; do
+    if [ "$arg" = "--debug" ]; then
+        DEBUG_BUILD=1
+    elif [ "$arg" = "--clean" ]; then
+        CLEAN_BUILD=1
+    fi
+done
+
+# Build using Makefile
+cd "$TEST_DIR"
+
+if [ "$CLEAN_BUILD" = "1" ]; then
+    echo ""
+    echo "Cleaning..."
+    make clean 2>/dev/null || true
+fi
+
+echo ""
+echo "Building..."
+if [ "$DEBUG_BUILD" = "1" ]; then
+    make DEBUG=1
+else
+    make
+fi
+
+echo ""
+echo "Build successful!"
+echo ""
+echo "Files in $OUTPUT_DIR:"
+ls -lh "$OUTPUT_DIR"
+echo ""
+echo "To test locally:"
+echo "  cd $PROJECT_ROOT/tests"
+echo "  npx serve apps"
+echo "  # Open http://localhost:3000/gal-webgl/gal_webgl_test.html"