Bootstrap stores + event bridge, add island hydration to boot.sx

- signals.sx: fix has? → has-key?, add def-store/use-store/clear-stores
  (L3 named stores), emit-event/on-event/bridge-event (event bridge)
- boot.sx: add sx-hydrate-islands, hydrate-island, dispose-island
  for client-side island hydration from SSR output
- bootstrap_js.py: add RENAMES, platform fns (domListen, eventDetail,
  domGetData, jsonParse), public API exports for all new functions
- bootstrap_py.py: add RENAMES, server-side no-op stubs for DOM events
- Regenerate sx-ref.js (with boot adapter) and sx_ref.py
- Update reactive-islands status: hydration, stores, bridge all spec'd

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

shared/sx/ref/signals.sx

@@ -307,13 +307,13 @@
   (fn (name init-fn)
     (let ((registry *store-registry*))
       ;; Only create the store once — subsequent calls return existing
-      (when (not (has? registry name))
+      (when (not (has-key? registry name))
         (set! *store-registry* (assoc registry name (init-fn))))
       (get *store-registry* name))))
 
 (define use-store
   (fn (name)
-    (if (has? *store-registry* name)
+    (if (has-key? *store-registry* name)
       (get *store-registry* name)
       (error (str "Store not found: " name
                   ". Call (def-store ...) before (use-store ...).")))))