Function spawn_subtree 

pub fn spawn_subtree(
    world: &mut World,
    el: &Element,
    parent: Option<Entity>,
    placeholders: Placeholders,
) -> Entity

Spawn an Element subtree under parent (or as a new root when None) WITHOUT touching the global LumenStylesheet resource.

Used by the dev-only devtools overlay mount: it applies its own isolated stylesheet to the overlay IR before spawning, so it must not clobber the app’s <for>-reconciler stylesheet the way LayoutIR::spawn_into does.