fix: ensure macOS mpv window helper is built and bundled correctly

- create dist/scripts dir before swiftc output in prepare-build-assets
- add after-pack verification that helper binary exists and is executable
- remove stale Windows PS1 extra-resource entry (harmless missing-file warnings)
- add tests for new verifyMacOSWindowHelper and prepare-build-assets behavior

scripts/prepare-build-assets.mjs

@@ -63,6 +63,8 @@ function buildMacosHelper() {
     return;
   }
 
+  ensureDir(scriptsOutputDir);
+
   try {
     execFileSync('swiftc', ['-O', macosHelperSourcePath, '-o', macosHelperBinaryPath], {
       stdio: 'inherit',