v0.17.10: bounded-cache writeback pipeline for big sequential writes

Pass 1 sweep speed on a healthy disc previously dipped from ~15 MB/s
to ~1 MB/s every ~30 s on a host with default Linux dirty-page
settings. Empirical cause: the kernel's vm.dirty_ratio (~20% of RAM)
lets hundreds of MB of dirty pages accumulate, then bursts a flush at
99% disk utilisation that blocks app writes for ~1 s. Confirmed on
the BU40N test bed — dirty pages grew 112 → 563 MB between bursts;
lowering vm.dirty_bytes to 64 MB at the host sysctl level eliminated
the dips. Shipping the equivalent inside libfreemkv so users do not
need to tune the host kernel.

- New crate::io::Writer: drop-in File wrapper (impl Write + Seek).
  Wraps a per-platform WritebackPipeline that on Linux schedules
  sync_file_range(WRITE) + lagging sync_file_range(WAIT_AFTER) +
  posix_fadvise(DONTNEED) in 32 MB chunks, bounding dirty cache at
  ~64 MB. macOS and Windows ship a no-op stub.
- Disc::sweep wraps its output File in Writer. Loop body unchanged.
- Module is purpose-built so any large sequential output (patch,
  mux) can adopt the same wrapper as a one-line change later.

src/io/writeback.rs

@@ -0,0 +1,17 @@
+//! Per-platform writeback pipeline. On Linux, drains dirty pages
+//! continuously at chunk granularity to keep the kernel's writeback
+//! queue bounded. On macOS and Windows, a no-op stub.
+//!
+//! The platform decision lives entirely in this file (the cfg-gated
+//! `pub use` below). Callers — and `DiskWriter` itself — are
+//! platform-independent.
+
+#[cfg(target_os = "linux")]
+mod linux;
+#[cfg(not(target_os = "linux"))]
+mod noop;
+
+#[cfg(target_os = "linux")]
+pub(super) use linux::WritebackPipeline;
+#[cfg(not(target_os = "linux"))]
+pub(super) use noop::WritebackPipeline;