Function apply_tiered_config 

pub fn apply_tiered_config(
    enabled: bool,
    total_budget: usize,
    backend: Backend,
    codec: Option<Codec>,
    swap_pageout: bool,
)

Apply a tiered-pager configuration. Reuses the singleton policy::TieredPolicy so in-flight ResidentTickets remain coherent with the running budget after the operator tunes any of the inputs.

When enabled is true, installs a ColumnPager backed by the singleton policy. When false, installs ColumnPager::disabled — in-flight tickets still credit the singleton, which is harmless: the budget grows above the configured total until the next enable reconciles it via reconfigure.

swap_pageout toggles MADV_PAGEOUT on the lz4 + swap spill path (see SWAP_PAGEOUT); it is stored unconditionally so the next page call observes it regardless of enabled.