Function discover 

pub fn discover(src_dir: &Path, cfg: &LumenToml) -> PagePlan

Discover the pages in src_dir (the app’s src/, from crate::app_layout::AppLayout), honouring [pages] / [app] config.

Single-file apps (a lone main.lmn or index.lmn, and no [pages] enabled = true) come back with multipage = false and the existing single-file load path runs unchanged - the compat guarantee.