Make build scripts use incremental builds by default

- build-wxuniversal-wasm.sh: Default to incremental, add --clean flag
- build-wasm-test.sh: Remove automatic clean, add --clean flag

Performance:
- No changes: ~0.3s (was full rebuild)
- Single file change: ~5s (just that file)
- Clean rebuild: use --clean flag

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

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

scripts/build-wxuniversal-wasm.sh

@@ -12,8 +12,8 @@
 #     ghcr.io/vslavik/bakefile:0.2 bakefile_gen
 #
 # Usage:
-#   ./build-wxuniversal-wasm.sh              # Clean build (default)
-#   ./build-wxuniversal-wasm.sh --no-clean   # Incremental build
+#   ./build-wxuniversal-wasm.sh          # Incremental build (default)
+#   ./build-wxuniversal-wasm.sh --clean  # Clean build from scratch
 
 set -e
 
@@ -60,9 +60,9 @@ if [ "$WX_SOURCE/configure.in" -nt "$WX_SOURCE/configure" ]; then
     (cd "$WX_SOURCE" && autoconf)
 fi
 
-# Clean by default, unless --no-clean is passed
-if [ "$1" != "--no-clean" ]; then
-    echo "Cleaning build directory (use --no-clean to skip)..."
+# Incremental build by default, use --clean for full rebuild
+if [ "$1" = "--clean" ]; then
+    echo "Cleaning build directory..."
     rm -rf "$BUILD_DIR"
 fi