iter11: WRITEBACK_CHUNK_BYTES 32 -> 128 MiB

Prediction: ~26-27 mean (regression from iter8). Confirms chunk-size
sweet spot at 32 MiB. Then move off chunk-size as a lever entirely.

src/io/writeback_file/mod.rs

@@ -73,8 +73,13 @@ use std::path::Path;
 use super::writeback::WritebackPipeline;
 
 /// Granularity at which the Linux writeback pipeline issues
-/// `sync_file_range` pairs. 32 MiB best-tested with iter8 (28.7 mean).
-const WRITEBACK_CHUNK_BYTES: u64 = 32 * 1024 * 1024;
+/// `sync_file_range` pairs.
+///
+/// iter11 (2026-05-17): 32 → 128 MiB. 0.21.14 tried this under Phase
+/// 2.5 and reverted; with Phase 2.5 disabled (iter8 baseline) the
+/// tradeoff is different. iter8 (32 MiB) = 28.7, iter9 (64 MiB) = 27.5.
+/// Trying 128 to see if the iter9 dip was noise or a real trend.
+const WRITEBACK_CHUNK_BYTES: u64 = 128 * 1024 * 1024;
 
 pub(crate) struct WritebackFile {
     file: File,