Island body: letrec instead of define (fixes render-to-dom), host-object JS fns

runner.sx: Converted define forms inside island body to letrec. Multiple
define forms in a let body cause render-to-dom to fall back to eval-expr
for the whole body, which evaluates (div ...) as a list instead of
rendering it to DOM. letrec keeps the last body expression (div) as the
render target.

sx_browser.ml: js_to_value now stores plain JS functions as host objects
(Dict with __host_handle) instead of wrapping as NativeFn. This preserves
the original JS function identity through the SX→JS round-trip, keeping
_driveAsync wrappers from host-callback intact when passed to
addEventListener via host-call.

Remaining: IO suspension in click handler is caught as "IO suspension in
non-IO context" instead of being driven by _driveAsync. The host-callback
wrapper creates the right JS function, but the event dispatch path doesn't
go through K.callFn.

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

hosts/ocaml/browser/sx_browser.ml

@@ -171,10 +171,14 @@ and js_to_value (js : Js.Unsafe.any) : value =
       if not (Js.Unsafe.equals h Js.undefined) then
         get_handle (Js.float_of_number (Js.Unsafe.coerce h) |> int_of_float)
       else
-        (* Plain JS function — wrap as NativeFn *)
-        NativeFn ("js-callback", fun args ->
-          let js_args = args |> List.map value_to_js |> Array.of_list in
-          js_to_value (Js.Unsafe.fun_call js (Array.map Fun.id js_args)))
+        (* Plain JS function — store as host object so value_to_js
+           returns the ORIGINAL JS function when passed to host-call.
+           This preserves wrappers like _driveAsync that host-callback
+           attaches for IO suspension handling. *)
+        let id = host_put js in
+        let d = Hashtbl.create 2 in
+        Hashtbl.replace d "__host_handle" (Number (float_of_int id));
+        Dict d
     | "object" ->
       let h = Js.Unsafe.get js (Js.string "__sx_handle") in
       if not (Js.Unsafe.equals h Js.undefined) then

sx/sx/applications/htmx/runner.sx

@@ -7,11 +7,9 @@
       (results (signal {}))
       (running (signal false))
       (current (signal "Ready")))
-    (define
-      get-doc
-      (fn () (host-get (dom-query "#test-iframe") "contentDocument")))
-    (define
-      wait-boot
+    (letrec
+      ((get-doc (fn () (host-get (dom-query "#test-iframe") "contentDocument")))
+        (wait-boot
           (fn
             ()
             (let
@@ -26,8 +24,7 @@
                     "true"))
                 true
                 (do (hs-wait 200) (wait-boot))))))
-    (define
-      reload-frame
+        (reload-frame
           (fn
             ()
             (let
@@ -36,8 +33,7 @@
               (hs-wait 1000)
               (wait-boot)
               (hs-wait 500))))
-    (define
-      run-action
+        (run-action
           (fn
             (action)
             (let
@@ -46,12 +42,16 @@
                 (= type :click)
                 (let
                   ((el (host-call doc "querySelector" (nth action 1))))
-              (when (nil? el) (error (str "Not found: " (nth action 1))))
+                  (when
+                    (nil? el)
+                    (error (str "Not found: " (nth action 1))))
                   (host-call el "click"))
                 (= type :fill)
                 (let
                   ((el (host-call doc "querySelector" (nth action 1))))
-              (when (nil? el) (error (str "Not found: " (nth action 1))))
+                  (when
+                    (nil? el)
+                    (error (str "Not found: " (nth action 1))))
                   (host-call el "focus")
                   (dom-set-prop el "value" (nth action 2))
                   (dom-dispatch el "input" nil)
@@ -61,7 +61,9 @@
                 (= type :assert-text)
                 (let
                   ((el (host-call doc "querySelector" (nth action 1))))
-              (when (nil? el) (error (str "Not found: " (nth action 1))))
+                  (when
+                    (nil? el)
+                    (error (str "Not found: " (nth action 1))))
                   (let
                     ((txt (host-get el "textContent"))
                       (kw (nth action 2))
@@ -69,7 +71,12 @@
                     (when
                       (and (= kw :contains) (not (contains? txt expected)))
                       (error
-                    (str "Expected '" expected "' in '" (slice txt 0 60) "'")))
+                        (str
+                          "Expected '"
+                          expected
+                          "' in '"
+                          (slice txt 0 60)
+                          "'")))
                     (when
                       (and (= kw :not-contains) (contains? txt expected))
                       (error (str "Unexpected '" expected "'")))))
@@ -85,8 +92,7 @@
                       (error (str "Expected >=" expected " got " count)))))
                 true
                 nil))))
-    (define
-      run-all
+        (run-all
           (fn
             ()
             (reset! running true)
@@ -108,7 +114,7 @@
                     (reset! results (assoc (deref results) name "pass")))))
               tests)
             (reset! running false)
-        (reset! current "Done")))
+            (reset! current "Done"))))
       (div
         (~tw :tokens "space-y-4")
         (div
@@ -170,7 +176,7 @@
           :id "test-iframe"
           :src "/sx/(applications.(htmx))"
           (~tw :tokens "w-full border border-stone-200 rounded-lg mt-4")
-        :style "height:600px"))))
+          :style "height:600px")))))
 
 ;; Page component — passes test definitions to the island
 (defcomp