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/ref/bootstrap_js.py

@@ -444,6 +444,8 @@ class JSEmitter:
             "dom-outer-html": "domOuterHtml",
             "dom-body-inner-html": "domBodyInnerHtml",
             "prevent-default": "preventDefault_",
+            "stop-propagation": "stopPropagation_",
+            "dom-focus": "domFocus",
             "element-value": "elementValue",
             "validate-for-request": "validateForRequest",
             "with-transition": "withTransition",
@@ -3461,6 +3463,8 @@ PLATFORM_ORCHESTRATION_JS = """
   // --- Events ---
 
   function preventDefault_(e) { if (e && e.preventDefault) e.preventDefault(); }
+  function stopPropagation_(e) { if (e && e.stopPropagation) e.stopPropagation(); }
+  function domFocus(el) { if (el && el.focus) el.focus(); }
   function elementValue(el) { return el && el.value !== undefined ? el.value : NIL; }
 
   function domAddListener(el, event, fn, opts) {