Constant PACK_FORMAT_VERSION 

pub const PACK_FORMAT_VERSION: u32 = 2; // 2u32

Current pack format version. Bumped when the on-disk layout changes in a non-additive way; older runtimes refuse newer packs by reading the version from the header.

v2 — adds 8 bytes to the header (flags + reserved) and per-entry compression (uncompressed_size, compression) in AssetSource::Packed. Older runtimes refuse v2 packs.