Constant WAL_FORMAT_VERSION 

pub const WAL_FORMAT_VERSION: u32 = 3;

The version the engine writes today. Phase 11.2 bumped 1 → 2 to Bumped 2 → 3 in Phase 11.9 to mark “may contain MVCC log-record frames” — frames whose page_num field carries crate::mvcc::MVCC_FRAME_MARKER (u32::MAX) instead of a real page number. v1 and v2 readers had no special-case for that marker and a pre-Phase-11.9 checkpoint would try to flush it to the main file at offset u32::MAX * PAGE_SIZE (way past EOF), which is why the bump is needed.