perf: reduce spawn_blocking contention in PUT path (#3132)

* perf: reduce spawn_blocking contention in PUT path (~23% throughput gain)

Flame graph profiling identified tokio blocking pool mutex contention
as the #1 bottleneck (17.3% of CPU time). Each spawn_blocking call must
acquire parking_lot::raw_mutex to enqueue work. With 16 concurrent PUTs
× 4 disks × 3+ spawn_blocking per disk, this became a serialization point.

Optimizations applied:
- Merge make_dir_all + file write into single spawn_blocking
- Merge read_file + parse + write + rename into single spawn_blocking
  for inline objects (small files)
- Optimize reliable_rename to try rename first, mkdir only on ENOENT
- Optimize remove/remove_std to try remove_file first, EISDIR fallback
- Add encode_inline_small fast path for small objects
- Parallelize bitrot writer creation with join_all

Benchmark (4KiB PUT, 4-disk EC, 16 concurrent, 8 rounds, randomized A/B):
  Baseline: ~950 obj/s → Optimized: ~1173 obj/s (+23%)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix: address review comments

- Return old_data_dir for non-inline rename_data path (was incorrectly None)
- Restore delete_all cleanup of PUT temp data on failure paths
- Fix cargo fmt formatting

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* style: apply rustfmt from stable 1.96.0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix: collapse nested if-let chains for clippy compliance

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix: address Copilot review comments

- fs.rs: handle macOS EPERM from remove_file on directories
- os.rs: restore NotFound=Ok(()) semantics on first rename attempt
- local.rs: use try-rename-then-mkdir pattern for inline rename_data

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* test: add unit tests for encode_inline_small fast path

* test: fix comment and line length in encode_inline_small tests

* fix: revert reliable_rename and write_all_internal to match original

Restore the original reliable_rename logic (check parent exists, then
rename in loop) and the original write_all_internal (make_dir_all outside
spawn_blocking). The optimization changes caused a CI-only test failure
in capacity_dirty_scope_test that could not be reproduced locally.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix: update comment and fix formatting for CI

- Fix macOS/BSD comment to accurately say macOS only
- Fix encode_inline_small test formatting to match rustfmt 1.96.0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix: propagate inline rename errors

* fix: retry rename when parent missing

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

crates/ecstore/src/disk/fs.rs

@@ -161,33 +161,39 @@ pub async fn make_dir_all(path: impl AsRef<Path>) -> io::Result<()> {
     fs::create_dir_all(path.as_ref()).await
 }
 
+fn is_dir_error(e: &io::Error) -> bool {
+    e.raw_os_error() == Some(libc::EISDIR)
+        || e.kind() == io::ErrorKind::IsADirectory
+        // macOS: remove_file on a directory returns EPERM
+        || (cfg!(target_os = "macos") && e.raw_os_error() == Some(libc::EPERM))
+}
+
 #[tracing::instrument(level = "debug", skip_all)]
 pub async fn remove(path: impl AsRef<Path>) -> io::Result<()> {
-    let meta = fs::metadata(path.as_ref()).await?;
-    if meta.is_dir() {
-        fs::remove_dir(path.as_ref()).await
-    } else {
-        fs::remove_file(path.as_ref()).await
+    // Try remove_file first; fall back to remove_dir if it's a directory
+    match fs::remove_file(path.as_ref()).await {
+        Ok(()) => Ok(()),
+        Err(e) if is_dir_error(&e) => fs::remove_dir(path.as_ref()).await,
+        Err(e) => Err(e),
     }
 }
 
 pub async fn remove_all(path: impl AsRef<Path>) -> io::Result<()> {
-    let meta = fs::metadata(path.as_ref()).await?;
-    if meta.is_dir() {
-        fs::remove_dir_all(path.as_ref()).await
-    } else {
-        fs::remove_file(path.as_ref()).await
+    // Try remove_file first; fall back to remove_dir_all if it's a directory
+    match fs::remove_file(path.as_ref()).await {
+        Ok(()) => Ok(()),
+        Err(e) if is_dir_error(&e) => fs::remove_dir_all(path.as_ref()).await,
+        Err(e) => Err(e),
     }
 }
 
 #[tracing::instrument(level = "debug", skip_all)]
 pub fn remove_std(path: impl AsRef<Path>) -> io::Result<()> {
-    let path = path.as_ref();
-    let meta = std::fs::metadata(path)?;
-    if meta.is_dir() {
-        std::fs::remove_dir(path)
-    } else {
-        std::fs::remove_file(path)
+    // Try remove_file first; fall back to remove_dir if it's a directory
+    match std::fs::remove_file(path.as_ref()) {
+        Ok(()) => Ok(()),
+        Err(e) if is_dir_error(&e) => std::fs::remove_dir(path.as_ref()),
+        Err(e) => Err(e),
     }
 }