Add baseline screenshots and visual regression testing

Add infrastructure for visual regression testing of wxWidgets WASM rendering:

- Add 121 baseline screenshots capturing all UI states tested by Playwright
- Add scripts/compare-screenshots.sh to compare test results vs baseline
- Update tests/WHATWORKS.md with documentation on screenshot comparison
- Update wxwidgets submodule with dialog refactoring

The baseline screenshots cover:
- Main app tabs (Controls, Text Input, Drawing, Lists, OpenGL, Grid, Dialogs)
- Standalone test apps (menu, toolbar, layout, aui, clipboard, filedialog, etc.)
- Dialog/MessageBox rendering (info, yes/no, error, custom dialogs)
- OpenGL rendering tests (immediate mode, vertex arrays, matrix ops)
- UI interactions (dropdowns, tab switching, z-ordering)

Use ./scripts/compare-screenshots.sh to check for visual regressions.
Small byte differences (<2%) are typically timestamps, not visual changes.

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

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

scripts/compare-screenshots.sh

@@ -0,0 +1,107 @@
+#!/bin/bash
+
+# Compare test screenshots with baseline screenshots
+# This script compares all PNG files in baseline-screenshots with test-results
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(dirname "$SCRIPT_DIR")"
+BASELINE_DIR="$PROJECT_ROOT/tests/baseline-screenshots"
+TEST_RESULTS_DIR="$PROJECT_ROOT/tests/test-results"
+
+# Check if directories exist
+if [ ! -d "$BASELINE_DIR" ]; then
+    echo "ERROR: Baseline directory not found: $BASELINE_DIR"
+    exit 1
+fi
+
+if [ ! -d "$TEST_RESULTS_DIR" ]; then
+    echo "ERROR: Test results directory not found: $TEST_RESULTS_DIR"
+    exit 1
+fi
+
+echo "=== Screenshot Comparison ==="
+echo "Baseline:     $BASELINE_DIR"
+echo "Test Results: $TEST_RESULTS_DIR"
+echo ""
+
+# Counters
+total=0
+identical=0
+different=0
+missing_current=0
+extra_current=0
+
+# Compare all baseline screenshots
+echo "=== Comparing Baseline Screenshots ==="
+for baseline in "$BASELINE_DIR"/*.png; do
+    filename=$(basename "$baseline")
+    current="$TEST_RESULTS_DIR/$filename"
+    total=$((total + 1))
+
+    if [ ! -f "$current" ]; then
+        echo "MISSING: $filename (not in test results)"
+        missing_current=$((missing_current + 1))
+        continue
+    fi
+
+    # Compare using cmp (byte-by-byte comparison)
+    if cmp -s "$baseline" "$current"; then
+        identical=$((identical + 1))
+        # Only show identical files if verbose
+        if [ "$1" = "-v" ] || [ "$1" = "--verbose" ]; then
+            echo "IDENTICAL: $filename"
+        fi
+    else
+        different=$((different + 1))
+
+        # Get file sizes
+        baseline_size=$(stat -f%z "$baseline" 2>/dev/null || stat -c%s "$baseline")
+        current_size=$(stat -f%z "$current" 2>/dev/null || stat -c%s "$current")
+        diff_bytes=$((current_size - baseline_size))
+
+        # Calculate percent difference
+        if [ "$baseline_size" -gt 0 ]; then
+            diff_pct=$(echo "scale=2; ($diff_bytes * 100) / $baseline_size" | bc)
+        else
+            diff_pct="N/A"
+        fi
+
+        echo "DIFFERENT: $filename (baseline: ${baseline_size}B, current: ${current_size}B, diff: ${diff_bytes}B / ${diff_pct}%)"
+    fi
+done
+
+# Check for extra files in test results not in baseline
+echo ""
+echo "=== Checking for Extra Screenshots ==="
+for current in "$TEST_RESULTS_DIR"/*.png; do
+    filename=$(basename "$current")
+    baseline="$BASELINE_DIR/$filename"
+
+    if [ ! -f "$baseline" ]; then
+        extra_current=$((extra_current + 1))
+        current_size=$(stat -f%z "$current" 2>/dev/null || stat -c%s "$current")
+        echo "EXTRA: $filename (${current_size}B, not in baseline)"
+    fi
+done
+
+# Summary
+echo ""
+echo "=== Summary ==="
+echo "Total baseline screenshots: $total"
+echo "Identical:                  $identical"
+echo "Different:                  $different"
+echo "Missing from test results:  $missing_current"
+echo "Extra in test results:      $extra_current"
+
+# Exit with error if there are differences
+if [ "$different" -gt 0 ] || [ "$missing_current" -gt 0 ]; then
+    echo ""
+    echo "WARNING: There are differences between baseline and test results!"
+    exit 1
+else
+    echo ""
+    echo "SUCCESS: All screenshots match baseline!"
+    exit 0
+fi