turbo-persistence: skip directory fsync on Windows (#95497)
Fixes #95495
### What
`TurboPersistence::commit()` syncs the database directory
(`File::open(&self.path)?.sync_data()?`) before updating CURRENT. On
Windows, `sync_data` calls `FlushFileBuffers`, which requires a handle
with write access and always fails with `ERROR_ACCESS_DENIED` on the
read-only directory handle `File::open` produces. Every commit fails,
persistence is disabled for the session, and dev memory eviction never
engages (details and a syscall-level demonstration are in #95495).
This skips the directory sync on Windows with `#[cfg(not(windows))]`.
The per-file `sync_data` calls are unaffected. If an equivalent
directory-entry durability guarantee is wanted on Windows it would need
a directory handle opened with write access; I kept this to the minimal
fix and can adjust if you prefer that approach. Skipping matches prior
art: LevelDB and SQLite do not sync directories on Windows either.
### Tests
No new test added: the failure is platform-specific and already covered
by the existing suite. On Windows, `cargo test -p turbo-persistence`
fails 30 of 60 tests before this change, all with `Access is denied. (os
error 5)`, and passes 60 of 60 with it (toolchain: nightly-2026-06-24,
x86_64-pc-windows-gnu, Windows 11). The directory sync was introduced in
#90542.
