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_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)"