Fix Windows shortcut updates and sign .node artifacts

- use `replace` when writing Windows mpv shortcuts and update tests
- include `*.node` files in SignPath Authenticode signing config
- document WinShell plugin sourcing in installer/build notes

src/main/runtime/windows-mpv-shortcuts.ts

@@ -77,7 +77,7 @@ export function applyWindowsMpvShortcuts(options: {
 
   const ensureShortcut = (shortcutPath: string): void => {
     mkdirSync(path.dirname(shortcutPath), { recursive: true });
-    const ok = options.writeShortcutLink(shortcutPath, 'create', details);
+    const ok = options.writeShortcutLink(shortcutPath, 'replace', details);
     if (!ok) {
       failures.push(shortcutPath);
     }