Fix hydration: effect was a no-op primitive, bytecode compiler emitted CALL_PRIM

Root cause: sx_primitives.ml registered "effect" as a native no-op (for SSR).
The bytecode compiler's (primitive? "effect") returned true, so it emitted
OP_CALL_PRIM instead of OP_GLOBAL_GET + OP_CALL. The VM's CALL_PRIM handler
found the native Nil-returning stub and never called the real effect function
from core-signals.sx.

Fix: Remove effect and register-in-scope from the primitives table. The server
overrides them via env_bind in sx_server.ml (after compilation), which doesn't
affect primitive? checks.

Also: VM CALL_PRIM now falls back to cek_call for non-NativeFn values (safety
net for any other functions that get misclassified).

15/15 source mode, 15/15 bytecode mode.

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

shared/static/wasm/sx/orchestration.sx

@@ -256,25 +256,6 @@
             "sx:afterSwap"
             (dict "target" target-el "swap" swap-style)))))))
 
-(define
-  flush-cssx!
-  :effects (mutation io)
-  (fn
-    ()
-    (let
-      ((rules (collected "cssx")))
-      (clear-collected! "cssx")
-      (when
-        (not (empty? rules))
-        (let
-          ((style (dom-query "#sx-css")))
-          (when
-            style
-            (dom-set-prop
-              style
-              "textContent"
-              (str (dom-get-prop style "textContent") (join "" rules)))))))))
-
 (define
   handle-sx-response
   :effects (mutation io)
@@ -527,8 +508,7 @@
     (sx-hydrate root)
     (sx-hydrate-islands root)
     (run-post-render-hooks)
-    (process-elements root)
-    (flush-cssx!)))
+    (process-elements root)))
 
 (define
   process-settle-hooks