Module engagement 

One-shot engagement reports for device routes.

The #1551 “GPU 0%” class: an Auto run that silently declines the device and falls back to the CPU otherwise leaves no trace of WHY. Every router reports the first engagement and the first decline it sees, once per process per route — the routes are per-minibatch or per-iterate, so an unconditional line would flood the fit log with thousands of identical entries. Routed through log::warn!, the repo’s sanctioned diagnostics path, so an initialised log backend lands it in the job logs.

Functions

note_route_engagement

Report route’s first engagement (engaged == true) or first decline (engaged == false, with fallback naming what runs instead — “falling back to CPU”, “CPU reference”); later calls with the same (route, engaged) are silent.