Revert "io/pipeline: bump WRITE_PIPELINE_DEPTH 16 → 32"

This reverts commit 95cb0ea935.

src/io/pipeline.rs

@@ -92,16 +92,10 @@ pub const DEFAULT_PIPELINE_DEPTH: usize = 4;
 /// consumer blocks on write.
 pub const READ_PIPELINE_DEPTH: usize = 32;
 
-/// Write pipeline depth. 0.21.8 restored the writer-thread + 128 MiB
-/// byte-bounded ring inside WritebackFile, which absorbs kernel
-/// writeback stalls downstream of this channel. The pre-0.21.8
-/// rationale ("smaller buffer reduces backpressure risk when
-/// sync_file_range blocks") no longer applies — `sync_file_range` only
-/// hits the writer thread, never this channel. Doubling 16 → 32 gives
-/// the matroska builder headroom to keep emitting frames when the sink
-/// momentarily lags behind, smoothing out the 5-50 MB/s instantaneous
-/// burst pattern observed on NFS bidirectional workloads at 0.21.8.
-pub const WRITE_PIPELINE_DEPTH: usize = 32;
+/// Write pipeline depth. Smaller buffer reduces backpressure risk when
+/// sync_file_range blocks; prevents producer from accumulating too much
+/// work while consumer waits for NFS to drain.
+pub const WRITE_PIPELINE_DEPTH: usize = 16;
 
 /// Channel depth for write-through pipelines. Each `send` fully
 /// drains before the next can enqueue. Use this when the producer