Hojun-Cho
cab85f5bc9
memio.fixedwrite returned a successful 0-byte write once the sink filled, so an overflowing fprintf/bsprintf surfaced a truncated prefix as a successful str instead of an error. Hare's fixed_write returns nomem there (ref/hare/memio/stream.ha:161); the bsprintf/fprintf io.error arm already forwards it, so the prefix-on-overflow path is the only divergence. Mirror Hare's full guard order: an empty input buf short-circuits to 0 (stream.ha:157) before the full-sink nomem guard, so a 0-byte write to a full sink stays 0 (no new divergence). fmt.bsprintf/formatone keep their logic; only their now-stale WHY-comments are rewritten, and formatone's tail-pad counter is left as-is (the width-form restore is a deferred follow-up, out of F-R scope). memio's own `fixed` doc comment, which still claimed ww surfaces 0 on a full buffer, is corrected to the new nomem contract. Tests: flip the two fmt rows that pinned the prefix bug (bsprintf_trunc, bsprintf_width_trunc) plus memiotest fixedwritecases' overflow row to assert `is nomem`; add positive controls (bsprintf_exact must still succeed) + an empty-sink discriminator (bsprintf_empty) + a dedicated fixedwritefull unit pinning the memio.ww:190 contract. Regenerates the w6c and wwdump combined.ww (memio's fixedwrite change and `fixed` doc comment are the only embedded changes; fmt is dead-code-eliminated from both).