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
(internal)/memory/0_18_redesign.md.

Single contributor: MattJackson.

src/mux/resolve.rs

@@ -242,23 +242,23 @@ pub fn output(
     match parsed {
         StreamUrl::Mkv { ref path } => {
             validate_file_path(path, "mkv")?;
-            // Wrap the raw `File` in `crate::io::Writer` (bounded-cache
+            // Wrap the output in `crate::io::WritebackFile` (bounded-cache
             // writeback) so a UHD-scale MKV mux to slow / network-attached
             // staging doesn't hit the dirty-page burst pathology that
             // sweep already side-steps. BufWriter sits on top to coalesce
             // mux's many small EBML element writes.
-            let file = std::fs::File::create(path)?;
             let writer: Box<dyn super::WriteSeek> = Box::new(std::io::BufWriter::with_capacity(
                 IO_BUF_SIZE,
-                crate::io::Writer::new(file)?,
+                crate::io::WritebackFile::create(path)?,
             ));
             Ok(Box::new(MkvStream::create(writer, title)?))
         }
         StreamUrl::M2ts { ref path } => {
             validate_file_path(path, "m2ts")?;
-            let file = std::fs::File::create(path)?;
-            let writer =
-                std::io::BufWriter::with_capacity(IO_BUF_SIZE, crate::io::Writer::new(file)?);
+            let writer = std::io::BufWriter::with_capacity(
+                IO_BUF_SIZE,
+                crate::io::WritebackFile::create(path)?,
+            );
             Ok(Box::new(M2tsStream::create(writer, title)?))
         }
         StreamUrl::Network { ref addr } => {