io+platform: phase 2 — sink trait split + fs_type detection

Introduces the SequentialSink / RandomAccessSink trait pair under
io::sink and an open_for_mkv dispatch helper that picks WritebackFile
on Linux+NFS and LocalFileSink everywhere else. LocalFileSink wraps
BufWriter<File> with a 4 MiB buffer and exposes a per-OS preallocate
path (fallocate on Linux, F_PREALLOCATE on macOS, no-op fallback).

Adds platform::fs_type::detect with a per-OS split (statfs on Linux /
macOS, UNC heuristic on Windows, Unknown elsewhere) so construction-
site dispatch has a single primitive to call.

Blanket impls cover the common shapes: any Write+Send is a
SequentialSink, and any SequentialSink+Seek is a RandomAccessSink.
WritebackFile satisfies the random-access trait via the blanket impl
without needing an explicit per-type impl. No callers wired yet — the
mux::resolve construction sites stay on WritebackFile pending Phase 3.

Tests: 5 new sink/preallocate tests + 3 fs_type tests (1 ignored,
needs a real NFS mount). cargo +1.86 fmt + clippy + tests all green.

src/io/sink/local_file.rs

@@ -0,0 +1,162 @@
+//! `LocalFileSink` — `BufWriter<File>` for the common local-disk case.
+//!
+//! Buffering: 4 MiB internal `BufWriter`. Sized to coalesce the small
+//! per-PES writes that come out of the muxer into kernel-page-aligned
+//! flushes without making the buffer big enough to matter for memory
+//! pressure on a single concurrent rip.
+//!
+//! `Seek` flushes the underlying `BufWriter` first; otherwise a seek
+//! could leapfrog buffered data and silently corrupt the file. This is
+//! the same shape `BufWriter` itself uses when it impls `Seek` in
+//! stdlib, and is necessary for MKV's seek-back operations (cluster
+//! size patch, Cues index, segment header backpatch) to land on the
+//! right offset.
+//!
+//! `RandomAccessSink` is satisfied via the blanket impl in
+//! [`super::mod`]; no explicit impl needed here.
+
+use std::fs::{File, OpenOptions};
+use std::io::{self, BufWriter, Seek, SeekFrom, Write};
+use std::path::Path;
+
+use super::preallocate;
+
+const BUFFER_BYTES: usize = 4 * 1024 * 1024;
+
+/// Random-access write sink for local disks.
+///
+/// Wraps a `BufWriter<File>` with a 4 MiB internal buffer and forwards
+/// `Write`/`Seek` so any call site that previously held a `File` or
+/// `WritebackFile` can drop this in. `finish()` flushes the buffer and
+/// runs `sync_all` on the underlying file so the caller can drop it
+/// without losing data.
+///
+/// Construction always opens the file `create + truncate + read +
+/// write`. `read` is enabled so the same handle can be reused for a
+/// verification re-read after the mux (the existing
+/// `FileSectorSink::create` pattern). On Linux, [`with_size_hint`]
+/// additionally calls `fallocate(FALLOC_FL_KEEP_SIZE)` to pre-reserve
+/// extents.
+///
+/// [`with_size_hint`]: Self::with_size_hint
+pub struct LocalFileSink {
+    inner: BufWriter<File>,
+}
+
+impl LocalFileSink {
+    /// Open `path` for writing, truncating any existing contents.
+    pub fn create(path: &Path) -> io::Result<Self> {
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .truncate(true)
+            .open(path)?;
+        Ok(Self {
+            inner: BufWriter::with_capacity(BUFFER_BYTES, file),
+        })
+    }
+
+    /// Like [`Self::create`] but additionally calls the per-OS
+    /// preallocate path with `size_bytes`. On Linux this is
+    /// `fallocate(FALLOC_FL_KEEP_SIZE)` so the on-disk extents are
+    /// reserved up front (reducing fragmentation for big sequential
+    /// muxer output); on other OSes it is a no-op today. Failures
+    /// from the preallocate call are non-fatal — the file is still
+    /// returned, just without the size reservation.
+    pub fn with_size_hint(path: &Path, size_bytes: u64) -> io::Result<Self> {
+        let file = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .truncate(true)
+            .open(path)?;
+        preallocate::preallocate(&file, size_bytes);
+        Ok(Self {
+            inner: BufWriter::with_capacity(BUFFER_BYTES, file),
+        })
+    }
+
+    /// Drain the internal buffer and `fsync` the underlying file.
+    /// Idempotent with `Drop` (the `BufWriter` also flushes on drop;
+    /// this call additionally surfaces fsync errors to the caller).
+    #[allow(dead_code)] // exposed for parity with WritebackFile::sync_all
+    pub fn sync_all(&mut self) -> io::Result<()> {
+        self.inner.flush()?;
+        self.inner.get_ref().sync_all()
+    }
+}
+
+impl Write for LocalFileSink {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.inner.write(buf)
+    }
+
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.inner.write_all(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.inner.flush()
+    }
+}
+
+impl Seek for LocalFileSink {
+    fn seek(&mut self, from: SeekFrom) -> io::Result<u64> {
+        // Flush before seeking so buffered bytes land at the offset
+        // they were written for, not the new one.
+        self.inner.flush()?;
+        self.inner.get_mut().seek(from)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::Read;
+
+    #[test]
+    fn write_seek_roundtrip() {
+        let dir = tempfile::tempdir().unwrap();
+        let p = dir.path().join("rt.bin");
+        let mut s = LocalFileSink::create(&p).unwrap();
+        s.write_all(b"AAAA").unwrap();
+        s.write_all(b"BBBB").unwrap();
+        // Seek back over the second word and overwrite.
+        s.seek(SeekFrom::Start(4)).unwrap();
+        s.write_all(b"CCCC").unwrap();
+        s.sync_all().unwrap();
+        drop(s);
+
+        let mut f = File::open(&p).unwrap();
+        let mut got = Vec::new();
+        f.read_to_end(&mut got).unwrap();
+        assert_eq!(&got[..], b"AAAACCCC");
+    }
+
+    #[test]
+    fn drop_flushes() {
+        let dir = tempfile::tempdir().unwrap();
+        let p = dir.path().join("drop.bin");
+        {
+            let mut s = LocalFileSink::create(&p).unwrap();
+            s.write_all(b"buffered").unwrap();
+            // No explicit flush / sync_all — BufWriter drop runs the
+            // flush and the file should land on disk.
+        }
+        let bytes = std::fs::read(&p).unwrap();
+        assert_eq!(&bytes[..], b"buffered");
+    }
+
+    #[test]
+    fn with_size_hint_creates_writable_file() {
+        let dir = tempfile::tempdir().unwrap();
+        let p = dir.path().join("sz.bin");
+        let mut s = LocalFileSink::with_size_hint(&p, 64 * 1024).unwrap();
+        s.write_all(b"hint-ok").unwrap();
+        s.sync_all().unwrap();
+        drop(s);
+        let bytes = std::fs::read(&p).unwrap();
+        assert_eq!(&bytes[..], b"hint-ok");
+    }
+}