v0.17.13: thread Writer through patch + mux for big-write consistency

The bounded-cache writeback wrapper (crate::io::Writer) was added in
0.17.10 and wired into Disc::sweep in 0.17.11, but the other two
paths in the crate that write large amounts of data sequentially —
Disc::patch and the MKV/M2TS mux — were still operating on raw
std::fs::File. That meant the dirty-page burst pathology the wrapper
exists to prevent could still bite on slow / network-attached staging
during recovery and mux phases.

This release plugs those gaps:

- Disc::patch (disc/mod.rs:1981) now wraps the reopened ISO in
  Writer before any seek / write. sync_all on Writer cleanly drains
  the in-flight chunk before the existing fsync.
- mux/resolve.rs MKV and M2TS branches wrap the output File in
  Writer underneath BufWriter. UHD MKV mux routinely produces 70+ GB
  of sequential output; the page cache no longer absorbs that as a
  single hot blast on slow targets.

Mapfile, log, settings, history, and stream-pipeline byte buffers
remain unchanged: those are either small one-shot writes (where
the wrapper has zero benefit and adds a stream_position syscall) or
already use bounded persistence (mapfile time-batched in 0.17.12).
The principle: any path that writes substantial sequential data to
a single file uses Writer; trivial writes don't.

src/mux/resolve.rs

@@ -242,15 +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
+            // 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, file));
+            let writer: Box<dyn super::WriteSeek> = Box::new(std::io::BufWriter::with_capacity(
+                IO_BUF_SIZE,
+                crate::io::Writer::new(file)?,
+            ));
             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, file);
+            let writer =
+                std::io::BufWriter::with_capacity(IO_BUF_SIZE, crate::io::Writer::new(file)?);
             Ok(Box::new(M2tsStream::create(writer, title)?))
         }
         StreamUrl::Network { ref addr } => {