Packed row data, RowCount * Dims meaningful elements
Dimensions per row
Changes when rows are renumbered (compaction, clear)
1 for a live row, 0 for a removed one
Live rows (RowCount minus tombstones)
Sum of squares of each row, precomputed so cosine needs only a dot product
Number of rows written, including tombstones
Process-unique identity of the store, stable for its lifetime
Changes on every mutation
Everything a reader on another thread needs to score a store, plus version stamps.