Store primitives + event-bridge + client? for isomorphic nav

- def-store/use-store/clear-stores: OCaml primitives with global
  mutable registry. Bypasses env scoping issues that prevented SX-level
  stores from persisting across bytecode module boundaries.

- client? primitive: _is_client ref (false on server, true in browser).
  Registered in primitives table for CALL_PRIM compatibility.

- Event-bridge island: rewritten to use document-level addEventListener
  via effect + host-callback, fixing container-ref timing issue.

- Header island: uses def-store for idx/shade signals when client? is
  true, plain signals when false (SSR compatibility).

- web-signals.sx: SX store definitions removed, OCaml primitives replace.

Isomorphic nav still fixme — client? works from K.eval but the JIT
"Not callable: nil" bug prevents proper primitive resolution during
render-to-dom hydration. Needs JIT investigation.

100 passed, 1 skipped, 0 failed.

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

web/web-signals.sx

@@ -35,25 +35,9 @@
 ;; Named stores — page-level signal containers
 ;; --------------------------------------------------------------------------
 
-(define *store-registry* (dict))
-
-(define def-store :effects [mutation]
-  (fn ((name :as string) (init-fn :as lambda))
-    (let ((registry *store-registry*))
-      (when (not (has-key? registry name))
-        (set! *store-registry* (assoc registry name (cek-call init-fn nil))))
-      (get *store-registry* name))))
-
-(define use-store :effects []
-  (fn ((name :as string))
-    (if (has-key? *store-registry* name)
-      (get *store-registry* name)
-      (error (str "Store not found: " name
-                  ". Call (def-store ...) before (use-store ...).")))))
-
-(define clear-stores :effects [mutation]
-  (fn ()
-    (set! *store-registry* (dict))))
+;; def-store, use-store, clear-stores are now OCaml primitives
+;; (sx_primitives.ml) with a global mutable registry that survives
+;; env scoping across bytecode modules and island hydration.
 
 
 ;; --------------------------------------------------------------------------