Function replay 

pub fn replay(
    pool: &DiffPool,
    rollback: &RollbackBuffer,
    start_tick: u64,
    end_tick: u64,
    apply_fn: impl FnMut((i32, i32, i32), u16, u16),
)

Replay diffs forward (old→new) to restore state after rewind.

For each diff in the frame range (oldest to newest), applies new_val.

apply_fn receives (coords, index, value_to_set) for each replay step.