Fqix::FormatVersion

On-disk format version, split into two orthogonal axes so each index kind can evolve on its own. major is the index kind (1 = sparse, 2 = exact), minor is a revision within that kind. Stored as two little-endian u16 fields, which is byte-compatible with the older single u32 version: the released exact value 2 decodes as 2.0, sparse 1 as 1.0.