regex string-pattern API + test-harness host-call-fn mock

Ported from loops/sx-vm-extensions 4ab9db05 + 8ec36b31.

- sx_primitives.ml (shared serving binary): regex-replace/split/match/etc. accept
  a raw pattern string (auto-compiled) as well as a compiled regex dict. Fixes
  (regex-replace "[0-9]" "_" s) / (regex-split "[ \t]+" s) which required a dict.
- run_tests.ml (test harness only): bind host-call-fn (= apply) + host-call-fn-raising
  / host-new-function / host-iter? / host-to-list in the mock DOM block. Recovers 712
  hyperscript behavioral tests that died on "Undefined symbol: host-call-fn"
  (run_tests --jit 1073 → 361 failures). No serving impact (test binary only).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_primitives.ml

@@ -2306,7 +2306,13 @@ let () =
           | Some r -> r
           | None -> raise (Eval_error "regex: handle not found"))
        | _ -> raise (Eval_error "regex: missing id"))
-    | _ -> raise (Eval_error "regex: expected regex dict")
+    | String s ->
+      (* Accept a raw pattern string: auto-compile it. Lets regex-split /
+         regex-match-all / regex-test / etc. take a pattern directly, not only
+         a pre-compiled regex dict from regex-compile. *)
+      (try (Re.compile (Re.Pcre.re s), s, "")
+       with _ -> raise (Eval_error ("regex: invalid pattern " ^ s)))
+    | _ -> raise (Eval_error "regex: expected regex dict or pattern string")
   in
   let group_to_dict g input =
     let d = Hashtbl.create 4 in
@@ -2368,6 +2374,14 @@ let () =
     | _ -> raise (Eval_error "regex-match-all: (regex string)"));
   register "regex-replace" (fun args ->
     match args with
+    | [String pattern; String replacement; String input] ->
+      (* string API: (regex-replace pattern replacement input) — replace ALL
+         matches of a raw pattern. Disambiguated from the compiled-regex API
+         below by the first arg being a String rather than a regex Dict. *)
+      (try
+        let re = Re.Pcre.re pattern |> Re.compile in
+        String (Re.replace_string re ~by:replacement input)
+       with _ -> String input)
     | [rx; String s; String replacement] ->
       let (re, _, flags) = regex_of_value rx in
       let expand g =