IO suspension driver: _driveAsync in platform, VmSuspended in value_to_js

- sx-platform.js: add _driveAsync to platform (was sandbox-only) for
  driving wait/fetch IO suspension chains in live site
- sx-platform.js: host-callback wrapper calls _driveAsync on callFn result
- sx_browser.ml: value_to_js callable wrapper catches VmSuspended, builds
  suspension object, and calls _driveAsync directly

Toggle and count clicks work fully. Bounce adds class but wait/remove
requires IO suspension in CEK context (eval-expr-cek doesn't support
perform — needs VM-path evaluation in hs-handler).

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

hosts/ocaml/browser/sx-platform.js

@@ -84,6 +84,31 @@
     }
   });
 
+  // IO suspension driver — resumes suspended callFn results (wait, fetch, etc.)
+  if (!window._driveAsync) {
+    window._driveAsync = function driveAsync(result) {
+      if (!result || !result.suspended) return;
+      var req = result.request;
+      var items = req && (req.items || req);
+      var op = items && items[0];
+      var opName = typeof op === "string" ? op : (op && op.name) || String(op);
+      var arg = items && items[1];
+      if (opName === "io-sleep" || opName === "wait") {
+        setTimeout(function() {
+          try { driveAsync(result.resume(null)); } catch(e) { console.error("[sx] driveAsync:", e.message); }
+        }, typeof arg === "number" ? arg : 0);
+      } else if (opName === "io-fetch") {
+        fetch(typeof arg === "string" ? arg : "").then(function(r) { return r.text(); }).then(function(t) {
+          try { driveAsync(result.resume({ok: true, text: t})); } catch(e) { console.error("[sx] driveAsync:", e.message); }
+        });
+      } else if (opName === "io-navigate") {
+        // navigation — don't resume
+      } else {
+        console.warn("[sx] unhandled IO:", opName);
+      }
+    };
+  }
+
   K.registerNative("host-callback", function(args) {
     var fn = args[0];
     // Native JS function — pass through
@@ -92,7 +117,9 @@
     if (fn && fn.__sx_handle !== undefined) {
       return function() {
         var a = Array.prototype.slice.call(arguments);
-        return K.callFn(fn, a);
+        var r = K.callFn(fn, a);
+        if (window._driveAsync) window._driveAsync(r);
+        return r;
       };
     }
     return function() {};