0.18 primitive: rename crate::io::Writer → WritebackFile

The type's job is the bounded-cache writeback pipeline (sync_file_range
+ posix_fadvise(DONTNEED)) — not generic writing. The 0.17 name was
ambiguous; reading `Writer::new(file)` gave no hint about what was
special. New name makes the role obvious at every call site.

Adds `WritebackFile::create(path)` and `WritebackFile::open(path)`
constructors so callers don't have to assemble a `File` first.

No alias kept; this is a clean 0.18 rename. See
freemkv-private/memory/0_18_redesign.md.

Single contributor: MattJackson.

src/io/mod.rs

@@ -1,16 +1,16 @@
 //! File I/O helpers that bound kernel cache pressure on big writes.
 //!
-//! `Writer` is a drop-in wrapper around `std::fs::File` for any call
-//! site that performs large sequential writes (sweep, mux, etc.). It
-//! implements `Write` and `Seek` so existing code paths can swap
-//! `File` for `Writer` with no body changes. Internally it drives a
-//! `WritebackPipeline` that, on Linux, drains dirty pages continuously
-//! at 32 MB granularity to avoid the kernel's accumulate-then-burst
-//! flush behaviour. macOS and Windows use a no-op pipeline — their
-//! default cache policies have not been shown to exhibit the same
-//! pathology for this access pattern.
+//! `WritebackFile` is a drop-in wrapper around `std::fs::File` for any
+//! call site that performs large sequential writes (sweep, patch, mux,
+//! etc.). It implements `Write` and `Seek` so existing code paths can
+//! swap `File` for `WritebackFile` with no body changes. Internally it
+//! drives a `WritebackPipeline` that, on Linux, drains dirty pages
+//! continuously at 32 MB granularity to avoid the kernel's
+//! accumulate-then-burst flush behaviour. macOS and Windows use a
+//! no-op pipeline — their default cache policies have not been shown
+//! to exhibit the same pathology for this access pattern.
 
 mod writeback;
-mod writer;
+mod writeback_file;
 
-pub(crate) use writer::Writer;
+pub(crate) use writeback_file::WritebackFile;