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