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/mod.rs

@@ -30,6 +30,7 @@
 pub(crate) mod bounded;
 pub mod byte_prefetcher;
 pub mod file_sector_source;
+pub mod fsync;
 pub mod sink;
 mod writeback;
 mod writeback_file;