Fix event-bridge + add client? primitive + header store foundation

- Event-bridge: rewrite island to use document-level addEventListener
  via effect + host-callback, bypassing broken container-ref + schedule-idle.
  Also use host-get for event-detail (WASM host handles).

- Add client? primitive: false on server (sx_primitives._is_client ref),
  true in browser (sx_browser.ml sets ref). Enables SSR-safe conditional
  logic for client-only features like def-store.

- Header island: use def-store for idx/shade signals when client? is true,
  falling back to plain signals on server. Foundation for SPA nav state
  preservation (store registry persistence still needs work).

- Remove unused client? K.eval override from sx-platform.js.

100 passed, 1 skipped (isomorphic nav — store registry resets on SPA nav), 0 failed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/reactive-islands/index.sx

@@ -515,16 +515,14 @@
 
 ;; 14. Event bridge — lake→island communication via custom DOM events
 (defisland ~reactive-islands/index/demo-event-bridge ()
-  (let ((container-ref (dict "current" nil))
-        (messages (signal (list)))
-        (_eff (schedule-idle (fn ()
-                (let ((el (get container-ref "current")))
-                  (when el
-                    (on-event el "inbox:message"
-                      (fn (e)
-                        (swap! messages (fn (old)
-                          (append old (host-get (event-detail e) "text"))))))))))))
-    (div :ref container-ref
+  (let ((messages (signal (list)))
+        (_eff (effect (fn ()
+                (let ((cb (host-callback
+                            (fn (e) (swap! messages (fn (old)
+                              (append old (host-get (event-detail e) "text"))))))))
+                  (host-call (dom-document) "addEventListener" "inbox:message" cb)
+                  (fn () (host-call (dom-document) "removeEventListener" "inbox:message" cb)))))))
+    (div
       (p :class "text-xs font-semibold text-stone-500 mb-2" "Event Bridge Demo")
       (p :class "text-sm text-stone-600 mb-2"
         "The buttons below simulate server-rendered content dispatching events into the island.")