Add Phase 2 P1 features: reactive class/style, refs, portals

- :class-map dict toggles classes reactively via classList.add/remove
- :style-map dict sets inline styles reactively via el.style[prop]
- ref/ref-get/ref-set! mutable boxes (non-reactive, like useRef)
- :ref attribute sets ref.current to DOM element after rendering
- portal render-dom form renders children into remote target element
- Portal content auto-removed on island disposal via register-in-scope
- Added #portal-root div to page shell template
- Added stop-propagation and dom-focus platform functions
- Demo islands for all three features on the demo page
- Updated status tables: all P0/P1 features marked Done

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/helpers.py

@@ -662,6 +662,7 @@ details.group{{overflow:hidden}}details.group>summary{{list-style:none}}details.
 <script type="text/sx" data-mount="body">{page_sx}</script>
 <script src="{asset_url}/scripts/sx-browser.js?v={sx_js_hash}"></script>
 <script src="{asset_url}/scripts/body.js?v={body_js_hash}"></script>
+<div id="portal-root"></div>
 </body>
 </html>"""