chore(ecstore): drop the disk dead_code blanket (#6139)

* chore(ecstore): drop the disk dead_code blanket

Removing the blanket exposes 36 items in the lowest storage layer: 7 deleted, 29 kept with reasoned item-level allows. That is the smallest deletion share of this burn-down, and the reason is a verification limit rather than a judgement call.

disk/local.rs carries 141 `#[cfg(target_os = "linux")]` sites — the densest platform gating in the tree, because O_DIRECT and io_uring only exist there. The direct-I/O cluster (six ENV_RUSTFS_OBJECT_DIRECT_IO_* constants plus is_direct_io_read_enabled, is_direct_io_write_enabled, get_direct_io_read_threshold, direct_write_staging_capacity, direct_write_tail_split and DIRECT_WRITE_STAGING_BYTES) reads as dead on macOS purely because its production callers at local.rs:1766, 3114 and 4605 sit inside Linux-gated blocks. direct_write_staging_capacity even documents itself as "Platform-independent (no O_DIRECT), so it is unit-tested on any host".

Deleting those would leave every local check green — 4096 tests pass, clippy is clean, make pre-commit exits 0 — and break the Linux build in CI, because all four local lanes compile for aarch64-apple-darwin. Cross-checking locally is not available either: cargo check --target x86_64-unknown-linux-gnu fails in the aws-lc-sys build script for want of a Linux C cross-compiler. Their allows name the platform reason so the next reader on a non-Linux host does not repeat the investigation.

Deleted, all in files with no target_os gating at all (os.rs, disk_store.rs):

- HealthDiskCtxKey and HealthDiskCtxValue with its private log_success. Note that DiskHealthTracker::log_success is a different method of the same name and is live from cluster/rpc/peer_s3_client.rs and remote_disk.rs — the two have to be told apart by type, not by name.
- LocalDiskWrapper::new_with_health and check_id.
- os.rs file_exists and lock_destination_directory_for_path_access.

Kept with allows: DiskHealthTracker's set_faulty, mark_offline, waiting_count and last_success have test callers in remote_disk.rs, so they only look dead in the lib target. to_disk_error, remove_all and sync_dir_files are asserted by their own files' tests. The reclaim, mmap and path-cache field groups are written but never read back.

Placement follows the same rule as the earlier roots: per-method allows inside impl DiskHealthTracker and impl LocalDisk, since both are mostly live and a block-level allow would be a smaller version of the blanket this issue removes. Struct-level allows are used only where the warning covers that struct's own fields. The three cached_read_env! functions take their allow inside the macro invocation, before the fn line, because the macro forwards $(#[$meta:meta])* onto the generated item.

Verification, four lanes warning-free: default, --tests, --features rio-v2 --tests, --features test-util --tests. cargo nextest run -p rustfs-ecstore 4096 passed; clippy --lib --tests -D warnings clean; make pre-commit exit 0. The Linux lane is not covered locally and is left to CI.

Ref rustfs/backlog#1823 (step 2).

* chore(ecstore): correct two dead_code reasons in the disk root

check_valid_path and reject_symlink_components have no caller at all -
not even a test - so 'asserted by this file's tests' misreads them as
covered. Both are method wrappers over live free functions; say that
instead.

Ref rustfs/backlog#1823.

crates/ecstore/src/disk/os.rs

@@ -571,6 +571,10 @@ fn regular_files(dir: &Path) -> io::Result<Vec<PathBuf>> {
 
 /// Fdatasync every regular file directly inside `dir`, then fsync the directory
 /// itself.
+#[allow(
+    dead_code,
+    reason = "reached only through sync_dir_files, whose callers are tests (backlog#1823)"
+)]
 pub fn sync_dir_files_std(dir: impl AsRef<Path>) -> io::Result<()> {
     for entry in std::fs::read_dir(dir.as_ref())? {
         let entry = entry?;
@@ -583,6 +587,7 @@ pub fn sync_dir_files_std(dir: impl AsRef<Path>) -> io::Result<()> {
 
 /// Async wrapper around [`sync_dir_files_std`]. Large directories flush files
 /// concurrently, bounded both per directory and process-wide.
+#[allow(dead_code, reason = "asserted by this file's tests (backlog#1823)")]
 pub async fn sync_dir_files(dir: impl AsRef<Path>) -> io::Result<()> {
     sync_dir_files_with_limiter(dir, Arc::new(Semaphore::new(MAX_PARALLEL_FILE_SYNCS))).await
 }
@@ -1809,10 +1814,6 @@ impl RenameCommitGuard {
         })
     }
 
-    pub(crate) fn lock_destination_directory_for_path_access(&self, directory: &Path) -> io::Result<RenameDestinationPathGuard> {
-        self.destination_directory_guard(directory, false)
-    }
-
     pub(crate) fn create_destination_directory_for_path_access(
         &self,
         directory: &Path,
@@ -2858,13 +2859,6 @@ pub async fn os_mkdir_all(dir_path: impl AsRef<Path>, base_dir: impl AsRef<Path>
     Ok(())
 }
 
-/// Check if a file exists.
-/// Returns true if the file exists, false otherwise.
-#[tracing::instrument(level = "debug", skip_all)]
-pub fn file_exists(path: impl AsRef<Path>) -> bool {
-    std::fs::metadata(path.as_ref()).map(|_| true).unwrap_or(false)
-}
-
 /// Whether an [`io::Error`] means "the directory is not empty".
 ///
 /// POSIX lets `rmdir`/`rename` report a non-empty directory as either