Function shared_pager 

pub fn shared_pager(enabled: bool) -> ColumnPager

A pager that, when enabled, draws from the shared tiered_policy budget pool — the same pool apply_tiered_config sizes for the process-global pager — and otherwise is a disabled (always-resident) pager.

This lets a second consumer (e.g. the storage upsert source stash) opt into the one shared budget independently of whether apply_tiered_config enabled the process-global pager for its own (compute) batchers. There is still a single budget pool and a single underlying mz_ore::pager; only the enable decision is per-consumer.