buildCsrTable

Builds the CSR table (ADR-2026-10-23-03). Phase A scatters the entries into 64 hash-high-bit partitions (per-segment histograms, a partition-major/segment-minor serial prefix - the argSortRadix digit-scatter pattern - preserving right-row order inside each partition). Phase B, one partition per worker, histograms its region's slots, prefixes into the global starts (each partition writes only its own slots' starts; the boundary belongs to the next partition, and starts[capacity] is written once at the end) and scatters the region into place - within a slot, the walk order is the region order, i.e. right-row order. Every written position is precomputed; nothing is shared and mutable across workers.