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/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);
+}