Fix Asyncify and improve build process

Build system improvements:
- Add wasm-opt stub for Docker build to bypass asyncify in container
- Add apply-asyncify.sh script for host-side asyncify transformation
- Update build-pcbnew.sh to install wasm-opt stub and add EXPORTED_RUNTIME_METHODS
- Add --with-zlib=sys to wxWidgets configure for proper zlib linking
- Improve update-baseline-screenshots.sh to only copy new/significantly changed files

wxWidgets submodule update:
- Fix EM_JS to EM_ASYNC_JS for proper Asyncify registration

Test updates:
- Add new baseline screenshots for bitmapbuttons, dnd, gridrenderers, printpreview, specialized tests
- Add kicad-pcbnew initial screenshot
- Fix wxgrid-controls.png and wxgrid-dedicated-page.png (were showing errors)

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

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

wasm/stubs/wasm-opt-stub.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+# Stub wasm-opt that does nothing
+# This allows Emscripten to generate Asyncify JS runtime without running the actual transformation
+# The real wasm-opt --asyncify is run on the host where more RAM is available
+
+# Handle --version query from Emscripten
+if [[ "$1" == "--version" ]]; then
+    echo "wasm-opt version 121 (version_121)"
+    exit 0
+fi
+
+# Find the output file argument (-o)
+OUTPUT=""
+INPUT=""
+PREV_ARG=""
+
+for arg in "$@"; do
+    if [ "$PREV_ARG" = "-o" ]; then
+        OUTPUT="$arg"
+    elif [[ "$arg" != -* ]] && [ -f "$arg" ]; then
+        INPUT="$arg"
+    fi
+    PREV_ARG="$arg"
+done
+
+# If we have both input and output, copy input to output
+if [ -n "$INPUT" ] && [ -n "$OUTPUT" ] && [ "$INPUT" != "$OUTPUT" ]; then
+    cp "$INPUT" "$OUTPUT"
+fi
+
+# Exit successfully so Emscripten thinks wasm-opt ran
+exit 0