Constant MVCC_FRAME_MARKER 

pub const MVCC_FRAME_MARKER: u32 = u32::MAX; // 4_294_967_295u32

Marker stored in the frame header’s page_num field that distinguishes MVCC log-record frames from page-commit frames. u32::MAX is safely outside the legal page-number range (max realistic database has at most a few hundred million pages, far short of u32::MAX).

Page-commit frames carry a real page number in [0, page_count); MVCC frames always carry this sentinel. The replayer branches on it.