io: add platform-aware fsync helpers (dir + durable file sync)

Add an io::fsync module with a per-OS split (posix/windows) mirroring the
writeback_file convention, replacing two duplicated dir-fsync copies:

- dir(): POSIX directory fsync; a no-op on Windows, where std cannot open
  a directory as a File and the failed open logged a spurious warning on
  every mapfile write.
- file_durable(): opens the target read+write before sync_all so the flush
  succeeds on Windows, where FlushFileBuffers rejects a read-only handle
  with ERROR_ACCESS_DENIED.

Point the mapfile writer at the shared dir() helper.

src/io/fsync/posix.rs

@@ -0,0 +1,18 @@
+//! POSIX directory-fsync. Active on unix and any non-Windows fallback target
+//! (BSD, illumos, …) — all share the same `File::open(dir).sync_all()`
+//! semantics. The Windows no-op lives in the sibling `windows` module.
+
+use std::path::Path;
+
+pub(super) fn fsync_dir(dir: &Path) {
+    match std::fs::File::open(dir) {
+        Ok(f) => {
+            if let Err(e) = f.sync_all() {
+                tracing::warn!(path = %dir.display(), error = %e, "failed to fsync directory");
+            }
+        }
+        Err(e) => {
+            tracing::warn!(path = %dir.display(), error = %e, "could not open directory to fsync");
+        }
+    }
+}