Splitting Correctness from Throughput: A Hybrid Approach to XLSX Streaming Writes

Writing XLSX files involves two concerns that have nothing to do with each other: OOXML correctness — relationships, content types, namespace declarations, theme references, drawing rels. Get these wrong and Excel may refuse to open the file or report a recovery dialog. The complexity is bounded but the surface is large. Per-cell throughput — 0 repeated millions of times. This dominates write time at scale. Yet most libraries solve both via a heavy object model (Apache POI's User Model — correct