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/signals.sx

@@ -306,7 +306,28 @@
 
 
 ;; ==========================================================================
-;; 12. Named stores — page-level signal containers (L3)
+;; 12. Refs — mutable boxes, no reactivity
+;; ==========================================================================
+;;
+;; A ref is a mutable container that does NOT trigger subscriptions when
+;; written. Like React's useRef: holds mutable values between renders, and
+;; provides imperative DOM element access via :ref attribute.
+
+(define ref
+  (fn (initial)
+    (dict "current" initial)))
+
+(define ref-get
+  (fn (r)
+    (get r "current")))
+
+(define ref-set!
+  (fn (r v)
+    (dict-set! r "current" v)))
+
+
+;; ==========================================================================
+;; 13. Named stores — page-level signal containers (L3)
 ;; ==========================================================================
 ;;
 ;; Stores persist across island creation/destruction. They live at page