HS tests: stringify DOM properties (innerHTML, textContent, value)

In a real browser, innerHTML/textContent/value are always strings.
The mock was storing raw SX values (Number, Bool, Nil), causing type
mismatches like "Expected 1, got 1" where the value was correct but
Number 1.0 != String "1".

Now coerces to string on host-set! for innerHTML, textContent, value,
outerHTML, innerText. Fixes 10 increment tests that were doing
`put value into me` with numeric results.

367 → 375/831 HS tests (+8 net, +10 new passes, -2 regressions).

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

hosts/ocaml/bin/run_tests.ml

@@ -1639,11 +1639,29 @@ let run_spec_tests env test_files =
        | _ -> (match Hashtbl.find_opt d (string_of_int i) with Some v -> v | None -> Nil))
     | _ -> Nil);
 
+  (* Stringify a value for DOM string properties *)
+  let dom_stringify = function
+    | String s -> String s
+    | Number n ->
+      let i = int_of_float n in
+      if float_of_int i = n then String (string_of_int i) else String (string_of_float n)
+    | Bool true -> String "true"
+    | Bool false -> String "false"
+    | Nil -> String ""
+    | v -> String (Sx_types.inspect v)
+  in
+
   reg "host-set!" (fun args ->
     match args with
     | [Nil; _; _] -> Nil
     | [Dict d; String key; value] ->
-      Hashtbl.replace d key value;
+      (* DOM string properties: coerce to string like a browser *)
+      let stored = match key with
+        | "innerHTML" | "textContent" | "outerHTML" | "value" | "innerText" ->
+          dom_stringify value
+        | _ -> value
+      in
+      Hashtbl.replace d key stored;
       (* Side effects for special keys *)
       (if key = "className" then
          match Hashtbl.find_opt d "classList" with
@@ -1651,7 +1669,7 @@ let run_spec_tests env test_files =
            (* classList sub-dict doesn't store classes — they live in className *)
            ignore cl
          | _ -> ());
-      value
+      stored
     | _ -> Nil);
 
   reg "host-call" (fun args ->