Wire reactive islands end-to-end: live interactive demos on the demo page

- Rebuild sx-browser.js with signals spec module (was missing entirely)
- Register signal functions (signal, deref, effect, computed, etc.) as
  PRIMITIVES so runtime-evaluated SX code in island bodies can call them
- Add reactive deref detection in adapter-dom.sx: (deref sig) in island
  scope creates reactive-text node instead of static text
- Add Island SSR support in html.py (_render_island with data-sx-island)
- Add Island bundling in jinja_bridge.py (defisland defs sent to client)
- Update deps.py to track Island dependencies alongside Component
- Add defisland to _ASER_FORMS in async_eval.py
- Add clear-interval platform primitive (was missing)
- Create four live demo islands: counter, temperature, imperative, stopwatch

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

shared/sx/ref/adapter-dom.sx

@@ -125,6 +125,13 @@
               ns
                 (render-dom-element name args env ns)
 
+              ;; deref in island scope → reactive text node
+              (and (= name "deref") *island-scope*)
+                (let ((sig-or-val (trampoline (eval-expr (first args) env))))
+                  (if (signal? sig-or-val)
+                    (reactive-text sig-or-val)
+                    (create-text-node (str (deref sig-or-val)))))
+
               ;; Fallback — evaluate then render
               :else
                 (render-to-dom (trampoline (eval-expr expr env)) env ns)))