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>

hosts/ocaml/lib/sx_primitives.ml

@@ -686,11 +686,15 @@ let () =
        | None -> raise (Eval_error ("Store not found: " ^ name)))
     | _ -> raise (Eval_error "use-store: expected (name)"));
   register "clear-stores" (fun _args -> Hashtbl.clear store_registry; Nil);
-  (* SSR stubs — effect is no-op on server (signals.sx guards with client?),
-     resource returns loading state. Other browser primitives only appear
-     inside effect bodies which never execute during SSR. *)
-  register "effect" (fun _args -> Nil);
-  register "register-in-scope" (fun _args -> Nil);
+  (* SSR stubs — resource returns loading state on server.
+     NOTE: effect and register-in-scope must NOT be registered as primitives
+     here — the bytecode compiler uses primitive? to decide CALL_PRIM vs
+     GLOBAL_GET+CALL. If effect is a primitive, bytecoded modules emit
+     CALL_PRIM which returns Nil instead of calling the real effect function
+     from core-signals.sx. The server overrides effect in sx_server.ml via
+     env_bind AFTER compilation. *)
+  (* register "effect" — REMOVED: see note above *)
+  (* register "register-in-scope" — REMOVED: see note above *)
   (* resource — SSR stub: return signal with {loading: true}, client hydrates real fetch *)
   register "resource" (fun _args ->
     let state = Hashtbl.create 8 in

hosts/ocaml/lib/sx_vm.ml

@@ -396,6 +396,8 @@ and run vm =
               in
               (match fn_val with
                | NativeFn (_, fn) -> fn args
+               | VmClosure _ | Lambda _ | Component _ | Island _ ->
+                 Sx_ref.cek_call fn_val (List args)
                | _ -> Nil)
             with Eval_error msg ->
               raise (Eval_error (Printf.sprintf "%s (in CALL_PRIM \"%s\" with %d args)"