HS: fix test-456 timeout + add sx_kernel_eval/hs_compile_inspect/hs_parse_inspect tools

- hs-run-filtered.js: add collectionExpressions to _NO_STEP_LIMIT_SUITES (fixes state
  corruption for downstream for-loop tests), add repeat-forever tests to _NO_STEP_LIMIT,
  extend slow deadline for collectionExpressions to 60s
- tests/hs-kernel-eval.js: new standalone Node.js eval script — full WASM kernel +
  mock DOM, accepts HS_EVAL_EXPR/MODE/SETUP/FILES env vars, supports eval/compile/parse modes
- tools/mcp_hs_test.py: add sx_kernel_eval, hs_compile_inspect, hs_parse_inspect tools
- hosts/ocaml/bin/mcp_tree.ml: add host_stubs param to sx_harness_eval (OCaml build pending)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/ocaml/bin/mcp_tree.ml

@@ -1892,8 +1892,34 @@ let handle_sx_harness_eval args =
     let file = args |> member "file" |> to_string_option in
     let setup_str = args |> member "setup" |> to_string_option in
     let files_json = try args |> member "files" with _ -> `Null in
+    let host_stubs = match args |> member "host_stubs" with `Bool b -> b | _ -> false in
     let e = !env in
     let warnings = ref [] in
+    (* Inject stub host primitives so files using host-get/host-new/etc. can load *)
+    if host_stubs then begin
+      let stubs = {|
+        (define host-global (fn (&rest _) nil))
+        (define host-get (fn (&rest _) nil))
+        (define host-set! (fn (obj k v) v))
+        (define host-call (fn (&rest _) nil))
+        (define host-new (fn (&rest _) (dict)))
+        (define host-callback (fn (f) f))
+        (define host-typeof (fn (&rest _) "string"))
+        (define hs-ref-eq (fn (a b) (identical? a b)))
+        (define host-call-fn (fn (&rest _) nil))
+        (define host-iter? (fn (&rest _) false))
+        (define host-to-list (fn (&rest _) (list)))
+        (define host-await (fn (&rest _) nil))
+        (define host-new-function (fn (&rest _) nil))
+        (define load-library! (fn (&rest _) false))
+      |} in
+      let stub_exprs = Sx_parser.parse_all stubs in
+      List.iter (fun expr ->
+        try ignore (Sx_ref.eval_expr expr (Env e))
+        with exn ->
+          warnings := Printf.sprintf "Stub warning: %s" (Printexc.to_string exn) :: !warnings
+      ) stub_exprs
+    end;
     (* Collect all files to load *)
     let all_files = match files_json with
       | `List items ->
@@ -3018,7 +3044,8 @@ let tool_definitions = `List [
      ("mock", `Assoc [("type", `String "string"); ("description", `String "Optional mock platform overrides as SX dict, e.g. {:fetch (fn (url) {:status 200})}")]);
      ("file", `Assoc [("type", `String "string"); ("description", `String "Optional .sx file to load for definitions")]);
      ("files", `Assoc [("type", `String "array"); ("items", `Assoc [("type", `String "string")]); ("description", `String "Multiple .sx files to load in order")]);
-     ("setup", `Assoc [("type", `String "string"); ("description", `String "SX setup expression to run before main evaluation")])]
+     ("setup", `Assoc [("type", `String "string"); ("description", `String "SX setup expression to run before main evaluation")]);
+     ("host_stubs", `Assoc [("type", `String "boolean"); ("description", `String "If true, inject nil-returning stubs for host-get/host-set!/host-call/host-new/etc. so files that use host primitives can load in the harness")])]
     ["expr"];
   tool "sx_nav" "Manage sx-docs navigation and articles. Modes: list (all nav items with status), check (validate consistency), add (create article + nav entry), delete (remove nav entry + page fn), move (move entry between sections, rewriting hrefs)."
     [("mode", `Assoc [("type", `String "string"); ("description", `String "Mode: list, check, add, delete, or move")]);