feat(webgl): Add GAL native test harness for visual regression testing

Add a standalone test application that compiles KiCad's actual OPENGL_GAL
against system wxWidgets to generate baseline screenshots for comparing
native OpenGL rendering against WebGL rendering in the browser.

Architecture:
- Compiles 18 KiCad GAL source files from the kicad submodule
- Uses template-based private member accessor (safer than #define private public)
- Generates shader C++ files from GLSL (10 shader pairs for SMAA AA)
- Minimal stubs for KiCad dependencies (PGM_BASE, ADVANCED_CFG, etc.)

Test coverage:
- 11 scenarios testing ~19% of GAL API
- basic-lines, line-widths, circles, arcs, rectangles, polygons
- alpha-blending, transforms, grid-cursor, segments, complex-scene

Key insights documented:
- GAL uses world-to-screen transformation requiring 1:1 pixel mapping
- FBO reading required for clean screenshots on macOS
- Layer depth needed for proper z-ordering in complex scenes

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

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

scripts/build-gal-native-test.sh

@@ -0,0 +1,71 @@
+#!/bin/bash
+#
+# Build script for the native GAL test harness
+#
+# This builds a standalone test application that uses KiCad's actual OPENGL_GAL
+# to render test scenarios and generate baseline screenshots.
+#
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+TEST_DIR="$PROJECT_ROOT/tests/gal-regression/native"
+BUILD_DIR="$TEST_DIR/build"
+LOG_FILE="$PROJECT_ROOT/tests/logs/gal-native-build.log"
+
+# Create log directory
+mkdir -p "$(dirname "$LOG_FILE")"
+
+echo "Building GAL Native Test..."
+echo "  Test dir: $TEST_DIR"
+echo "  Build dir: $BUILD_DIR"
+echo "  Log file: $LOG_FILE"
+
+# Create build directory
+mkdir -p "$BUILD_DIR"
+cd "$BUILD_DIR"
+
+# Generate shader headers if needed
+if [ ! -d "$TEST_DIR/generated" ] || [ -z "$(ls -A $TEST_DIR/generated 2>/dev/null)" ]; then
+    echo "Generating shader headers..."
+    python3 "$TEST_DIR/generate_shaders.py" >> "$LOG_FILE" 2>&1
+fi
+
+# Configure
+echo "Running CMake..."
+cmake .. >> "$LOG_FILE" 2>&1
+CMAKE_STATUS=$?
+
+if [ $CMAKE_STATUS -ne 0 ]; then
+    echo "CMake configuration failed! Check $LOG_FILE for details."
+    echo ""
+    echo "Last 50 lines of log:"
+    tail -50 "$LOG_FILE"
+    exit 1
+fi
+
+# Build
+echo "Building..."
+make -j$(sysctl -n hw.ncpu 2>/dev/null || echo 4) >> "$LOG_FILE" 2>&1
+BUILD_STATUS=$?
+
+if [ $BUILD_STATUS -ne 0 ]; then
+    echo "Build failed! Check $LOG_FILE for details."
+    echo ""
+    echo "Last 100 lines of log:"
+    tail -100 "$LOG_FILE"
+    exit 1
+fi
+
+echo "Build successful!"
+echo "Executable: $BUILD_DIR/gal_native_test"
+echo ""
+echo "To run:"
+echo "  $BUILD_DIR/gal_native_test"
+echo ""
+echo "Options:"
+echo "  --output <dir>   Output directory for baseline PNGs"
+echo "  --width <w>      Canvas width (default: 800)"
+echo "  --height <h>     Canvas height (default: 600)"
+echo "  --show           Show window (default: headless)"