Function spawn_warmup 

pub fn spawn_warmup(no_vello: bool) -> Option<JoinHandle<()>>

Spawn a detached background worker that constructs a throw-away FontSystem, shapes CANONICAL_STRINGS against it, then — unless no_vello is set — builds a 4×4 scratch crate::offscreen::OffscreenRenderer and renders one solid-colour scene.

Dropping the returned JoinHandle before the worker finishes doesn’t cancel anything — the process keeps whatever warmup already completed. Returns None when warmup is disabled via TILO_STARTUP_WARM=off.