Function charge_for 

pub fn charge_for(
    state: PageState,
    kind: PageKind,
    is_write: bool,
) -> Result<(u64, PageState), HardFault>

The category-#3 charge and resulting state for one page touch.

Read-only page-in is free at the fault — the cost of bringing a region into the working set is charged eagerly at the CALL that maps it (crate::gas_const::call_frame_cost). Copy-on-write (the first write to a writable page) is the only fault-driven #3 charge:

• first read (NotPresent) → 0, → PresentRo
• first write (NotPresent) → COW_COST, → PresentRw
• write after read (PresentRo) → COW_COST, → PresentRw
• already present for this access kind → 0
• write to PinnedCapRo → HardFault