Function dev_page_html 

pub fn dev_page_html(source: &str, stem: &str) -> String

The document largo serve sends for a page — the same pre-rendered markup largo build ships, so what a developer looks at is what their visitors get.

Public so mime_single_source_lock holds dev and prod against each other through the REAL functions. They diverged before: the build pre-rendered the content and the dev server served the bare shell.