Segments and extents differ deliberately

Two scales, two measurements. A segment file answers to file management — K3's 22.61 GiB routed layer exceeds the 20 GiB cap, so it ships as two segments of 448 experts. A group extent answers to hardware — the unit one grouped dispatch reads. Boundaries always agree: segments split only on extent edges, and conflating the scales would turn a two-file layer into fourteen for no read benefit.