A 1 MiB BufWriter sits above glibc's 128 KiB mmap threshold; depending on
allocator history it was mapped afresh by every write, and its page faults
(1.6 M vs 13 K over the criterion write_2d_chunked group) made 1 MiB
chunked writes 1.35x-1.83x slower than main. With 64 KiB they match main.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
