Function on_run_retrack 

pub fn on_run_retrack(f: impl FnOnce() + 'static) -> bool

Hang f on the active computation’s current run: it fires when that computation next re-tracks (a re-run clears its sources first) or is disposed. This is the hook that lets an external dependency registry (day-model’s trigger claims) mirror this graph’s per-run source bookkeeping. Returns false, registering nothing, when no computation is tracking.