WIP: bytecode when/do/perform — host-callback _driveAsync fix + debugging

Root cause identified: nested cek_call_or_suspend calls on same VM
overwrite pending_cek. First call suspends (thunk's hs-wait), second
call from synchronous dom-listen callback overwrites before resume.

sandbox host-callback: removed _driveAsync call to prevent duplicate
resume chains. Still 3/6 in Node.js test — issue is in OCaml call
stack nesting, not JS async.

Next: prevent pending_cek overwrite in nested CEK→VM→CEK→VM chains.

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

tests/playwright/sx-inspect.js

@@ -1391,9 +1391,10 @@ async function modeSandbox(page, expr, files, setup, stack, bytecode) {
       if (typeof fn === 'function' && fn.__sx_handle === undefined) return fn;
       if (fn && fn.__sx_handle !== undefined) {
         return function() {
-          const r = K.callFn(fn, Array.from(arguments));
-          window._driveAsync(r);
-          return r;
+          // Do NOT call _driveAsync here — the suspension propagates through
+          // the normal CEK/VM return path. The outer callFn handles it.
+          // Calling _driveAsync would create a duplicate resume chain.
+          return K.callFn(fn, Array.from(arguments));
         };
       }
       return function() {};