FileBuilder::write: 64 KiB write buffer, not 1 MiB

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]>

crates/clawhdf5/src/writer.rs

@@ -289,7 +289,12 @@ fn write_file_atomically_with<E: From<std::io::Error>>(
     let tmp_path = path.with_file_name(tmp_name);
 
     let result = (|| -> Result<(), E> {
-        let mut f = std::io::BufWriter::with_capacity(1 << 20, std::fs::File::create(&tmp_path)?);
+        // 64 KiB, below glibc's 128 KiB mmap threshold: a 1 MiB buffer was
+        // mapped afresh by each write once the threshold had not risen, and
+        // its page faults made a 1 MiB chunked write 1.6x slower (criterion
+        // `write_2d_chunked/512x512` after the smaller cases, 2026-09-29).
+        // Writes larger than the buffer go straight to the file.
+        let mut f = std::io::BufWriter::with_capacity(64 << 10, std::fs::File::create(&tmp_path)?);
         write(&mut f)?;
         f.flush()?;
         f.get_ref().sync_all()?;