Fix callable? type mismatch, restore 20 HS test regressions, add host-* server stubs

callable? in boot-helpers.sx checked for "native-fn" but type-of returns
"function" for NativeFn — broke make-spread and all native fn dispatch
in aser. Restore 20 behavioral tests replaced with NOT IMPLEMENTED stubs
by the test regeneration commit. Add host-* platform primitive stubs to
sx_server.ml so boot-helpers.sx loads without errors server-side.

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

hosts/ocaml/bin/sx_server.ml

@@ -368,6 +368,9 @@ let cek_run_with_io state =
   loop ()
 
 (** IO-aware eval_expr — like eval_expr but handles IO suspension. *)
+(* IO-aware eval — used by site_mode. The regular file loading path
+   uses Sx_ref.eval_expr which delegates IO suspension to the
+   _cek_io_suspend_hook → _vm_suspension_to_dict chain. *)
 let _eval_expr_io expr env =
   let state = Sx_ref.make_cek_state expr env (List []) in
   cek_run_with_io state

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-da652b03",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-def18509",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-875d5bae",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-def18509",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new