Function diff_filler_provider_id 

pub fn diff_filler_provider_id(buffer_id: BufferId, side: Side) -> ProviderId

D.4.c: deterministic [ProviderId] for the filler-row provider on buffer_id and side. Exposed as a free function so :diffoff / D.4.d teardown can unregister without holding the provider — the namespace encoding makes the id reproducible.