Constant WAL_FORMAT_VERSION_MIN_SUPPORTED 

pub const WAL_FORMAT_VERSION_MIN_SUPPORTED: u32 = 1;

Lowest format version we know how to open. v1 had the bytes that now hold clock_high_water reserved-as-zero, which is identical to “clock has never been persisted” and round-trips cleanly. v2 adds the clock_high_water field but no MVCC frames. v3 adds MVCC frames; downgrading a v3 WAL into a v2 reader would mis-handle the MVCC marker page number — but a fresh truncate (every checkpoint) rewrites the header at the engine’s current version, so the cross-version exposure is bounded.