Add a speculative parser that runs on a background thread to discover preloadable resources while the main parser is blocked on script execution. - PreloadScanner: lightweight state machine that identifies <link rel=stylesheet>, <script src>, <img src>, and <link rel=preload> tags - SpeculativeParser: manages background thread, mpsc channels, token buffering, and invalidation on document.write - Handles inline script content correctly (enters ScriptData mode) - Skips HTML comments to avoid false positives - Supports double-quoted, single-quoted, and unquoted attribute values - 22 new tests covering resource discovery, speculation hit/miss, invalidation, and token replay Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>