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/preallocate/linux.rs

@@ -0,0 +1,20 @@
+//! Linux `fallocate(FALLOC_FL_KEEP_SIZE)` preallocation.
+//!
+//! `KEEP_SIZE` reserves extents without changing the apparent file
+//! length, which matches the muxer's expectation that writes still grow
+//! the file naturally.
+
+use std::fs::File;
+#[cfg(unix)]
+use std::os::unix::io::AsRawFd;
+
+pub(super) fn preallocate_impl(file: &File, size_bytes: u64) {
+    let fd = file.as_raw_fd();
+    // FALLOC_FL_KEEP_SIZE = 0x01.
+    let rc = unsafe { libc::fallocate(fd, libc::FALLOC_FL_KEEP_SIZE, 0, size_bytes as i64) };
+    tracing::debug!(
+        target: "mux",
+        "LocalFileSink fallocate size_hint={size_bytes} rc={rc} ok={}",
+        rc == 0
+    );
+}

src/io/sink/preallocate/macos.rs

@@ -0,0 +1,48 @@
+//! macOS `F_PREALLOCATE` extent reservation.
+//!
+//! `fcntl(F_PREALLOCATE)` with `F_ALLOCATECONTIG` first (try for a
+//! contiguous run) and fall back to `F_ALLOCATEALL` (non-contig OK).
+//! Reported file size is unchanged — the muxer's writes still grow it.
+
+use std::fs::File;
+use std::os::unix::io::AsRawFd;
+
+// Mirror the Darwin `fstore_t` struct from `<sys/fcntl.h>`. libc on
+// some Rust toolchains/versions doesn't ship this binding, so define
+// it locally with the layout the kernel ABI requires.
+#[repr(C)]
+struct Fstore {
+    fst_flags: libc::c_uint,
+    fst_posmode: libc::c_int,
+    fst_offset: libc::off_t,
+    fst_length: libc::off_t,
+    fst_bytesalloc: libc::off_t,
+}
+
+// Constants from <sys/fcntl.h>.
+const F_PREALLOCATE: libc::c_int = 42;
+const F_ALLOCATECONTIG: libc::c_uint = 0x0000_0002;
+const F_ALLOCATEALL: libc::c_uint = 0x0000_0004;
+const F_PEOFPOSMODE: libc::c_int = 3;
+
+pub(super) fn preallocate_impl(file: &File, size_bytes: u64) {
+    let fd = file.as_raw_fd();
+    let mut store = Fstore {
+        fst_flags: F_ALLOCATECONTIG,
+        fst_posmode: F_PEOFPOSMODE,
+        fst_offset: 0,
+        fst_length: size_bytes as libc::off_t,
+        fst_bytesalloc: 0,
+    };
+    let mut rc = unsafe { libc::fcntl(fd, F_PREALLOCATE, &mut store as *mut Fstore) };
+    if rc == -1 {
+        // Fall back to non-contiguous.
+        store.fst_flags = F_ALLOCATEALL;
+        rc = unsafe { libc::fcntl(fd, F_PREALLOCATE, &mut store as *mut Fstore) };
+    }
+    tracing::debug!(
+        target: "mux",
+        "LocalFileSink F_PREALLOCATE size_hint={size_bytes} rc={rc} bytesalloc={}",
+        store.fst_bytesalloc
+    );
+}

src/io/sink/preallocate/mod.rs

@@ -0,0 +1,27 @@
+//! Per-OS extent preallocation. Best-effort; failures are logged at
+//! debug and otherwise swallowed because the file is still usable
+//! without the size reservation — only large-file fragmentation gets
+//! marginally worse.
+
+use std::fs::File;
+
+#[cfg(target_os = "linux")]
+mod linux;
+#[cfg(target_os = "macos")]
+mod macos;
+#[cfg(not(any(target_os = "linux", target_os = "macos")))]
+mod other;
+
+#[cfg(target_os = "linux")]
+use linux::preallocate_impl;
+#[cfg(target_os = "macos")]
+use macos::preallocate_impl;
+#[cfg(not(any(target_os = "linux", target_os = "macos")))]
+use other::preallocate_impl;
+
+/// Reserve `size_bytes` of disk space for `file`'s on-disk extents.
+/// Reported file size is unchanged — writes still grow the file
+/// naturally; only the allocator's extent map is primed.
+pub(super) fn preallocate(file: &File, size_bytes: u64) {
+    preallocate_impl(file, size_bytes);
+}

src/io/sink/preallocate/other.rs

@@ -0,0 +1,10 @@
+//! Fallback preallocate impl. No-op.
+
+use std::fs::File;
+
+pub(super) fn preallocate_impl(_file: &File, size_bytes: u64) {
+    tracing::debug!(
+        target: "mux",
+        "LocalFileSink preallocate size_hint={size_bytes} skipped (no platform impl)"
+    );
+}