Unify CALL_PRIM dispatch: vm_globals as single source of truth

Seed all primitives into vm_globals as NativeFn values at init.
CALL_PRIM now looks up vm.globals only (not the separate primitives
table). This means OP_DEFINE and registerNative naturally override
primitives — browser.sx's (define set-cookie ...) now takes effect.

The primitives Hashtbl remains for the compiler's primitive? predicate
but has no runtime dispatch role.

Tests: 2435 pass / 64 fail (pre-existing), vs 1718/771 baseline.

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

hosts/ocaml/bin/sx_server.ml

@@ -680,6 +680,14 @@ let () =
     if env.parent = None then
       Hashtbl.replace _shared_vm_globals name v)
 
+(* Seed _shared_vm_globals with all primitives as NativeFn values.
+   Makes vm_globals the single source of truth for CALL_PRIM dispatch. *)
+let () =
+  Hashtbl.iter (fun name fn ->
+    if not (Hashtbl.mem _shared_vm_globals name) then
+      Hashtbl.replace _shared_vm_globals name (NativeFn (name, fn))
+  ) Sx_primitives.primitives
+
 let make_server_env () =
   let env = make_env () in
   Sx_render.setup_render_env env;

hosts/ocaml/browser/sx_browser.ml

@@ -857,6 +857,15 @@ let () =
 
 let () = ignore (env_bind global_env "enable-jit!" (NativeFn ("enable-jit!", fun _ -> _jit_enabled := true; Nil)))
 
+(* Seed _vm_globals with all primitives as NativeFn values.
+   This makes _vm_globals the single source of truth for CALL_PRIM dispatch.
+   OP_DEFINE and registerNative naturally override entries here. *)
+let () =
+  Hashtbl.iter (fun name fn ->
+    if not (Hashtbl.mem _vm_globals name) then
+      Hashtbl.replace _vm_globals name (NativeFn (name, fn))
+  ) Sx_primitives.primitives
+
 (* ================================================================== *)
 (* Register global SxKernel object                                     *)
 (* ================================================================== *)

hosts/ocaml/lib/sx_vm.ml

@@ -387,11 +387,10 @@ and run vm =
             | _ -> v) args in
           let result =
             try
-              (* Check primitives FIRST (native implementations of map/filter/etc.),
+              (* Single lookup: vm.globals is the sole source of truth.
-                 then globals (which may have ho_via_cek wrappers that route
+                 Primitives are seeded into vm.globals at init as NativeFn values.
-                 through the CEK — these can't call VM closures). *)
+                 OP_DEFINE and registerNative naturally override them. *)
-              let fn_val = try Sx_primitives.get_primitive name with _ ->
+              let fn_val = try Hashtbl.find vm.globals name with Not_found ->
-                try Hashtbl.find vm.globals name with Not_found ->
                   raise (Eval_error ("VM: unknown primitive " ^ name))
               in
               (match fn_val with