The previous fix only guarded domAppend/domInsertAfter, but many
platform JS functions (asyncRenderChildren, asyncRenderElement,
asyncRenderMap, render, sxRenderWithEnv) call appendChild directly.
Add _spread guards to all direct appendChild sites. For async element
rendering, merge spread attrs onto parent (class/style join, others
overwrite) matching the sync adapter behavior.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>