Introduces `PersistentSparseCompactIntMatrix`, a row-major sparse integer matrix combining a bitmatrix support layer with dual value streams for singleton and multi partitions. Adds builder and packing utilities to convert dense count matrices into the new on-disk format, featuring idempotency guards and crash-safe persistence workflows. Exposes core accessors, iterators, and column weight calculations while adjusting internal visibility for companion modules. Includes unit tests and diagnostic examples to validate equivalence with dense representations and measure compaction metrics.
