cache-bust: add filemtime-based versioning to all CSS and JS assets

src/App.php

@@ -136,6 +136,19 @@ class App
         exit;
     }
 
+    // ── Asset versioning ─────────────────────────────────────────────────────
+
+    /**
+     * Return an asset URL with a filemtime-based cache-busting query string.
+     * Input is a root-relative URL path (e.g. /assets/css/main.css).
+     */
+    public static function assetV(string $urlPath): string
+    {
+        $file = APP_ROOT . '/public' . $urlPath;
+        $v    = file_exists($file) ? filemtime($file) : 0;
+        return $urlPath . ($v ? '?v=' . $v : '');
+    }
+
     // ── Template rendering ────────────────────────────────────────────────────
 
     /**