Constant PAGE_IN_COST 

pub const PAGE_IN_COST: u64 = 64;

#3 read-only page-in, charged per declared 2 MiB unit at the CALL (no longer per touched unit at a fault): the cost of admitting one read-only unit — code or a pinned DataCap intersected with a 2 MiB cluster — into the working set. Folded into call_frame_cost; a read at a fault charges nothing. TODO(gas-calibration): placeholder, subject to change.