Constant MAX_DISK_BYTES 

const MAX_DISK_BYTES: u64 = _; // 8_388_608u64

Cap on disk file size before rotation. 8 MiB ≈ 4–8K events depending on event variant. Two-file rotation: when current exceeds this, rename to .old (overwriting any prior .old) and start fresh. Total worst-case footprint: 2 × MAX_DISK_BYTES.