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>

sx/sx/reactive-islands.sx

@@ -111,8 +111,8 @@
               (td :class "px-3 py-2 font-mono text-xs text-stone-500" "signals.sx: emit-event, on-event, bridge-event"))
             (tr :class "border-b border-stone-100"
               (td :class "px-3 py-2 text-stone-700" "Client hydration")
-              (td :class "px-3 py-2 text-amber-600 font-medium" "TODO")
-              (td :class "px-3 py-2 font-mono text-xs text-stone-500" "boot.sx"))
+              (td :class "px-3 py-2 text-green-700 font-medium" "Spec'd")
+              (td :class "px-3 py-2 font-mono text-xs text-stone-500" "boot.sx: sx-hydrate-islands, hydrate-island, dispose-island"))
             (tr :class "border-b border-stone-100"
               (td :class "px-3 py-2 text-stone-700" "Event bindings")
               (td :class "px-3 py-2 text-amber-600 font-medium" "TODO")