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;