Function env_port_override 

pub fn env_port_override(program_port: u16) -> u16

The port a Serve statement actually binds: the LOGOS_PORT environment variable when set (falling back to PORT, the Fly/Heroku/Render convention), else the program’s own port expression. ONE seam serving both the largo full-stack dev proxy (which runs the program as a child on a port it chose) and PaaS deployment — implemented here so the interpreter and AOT tiers cannot drift.