Revert "perf(blob): one syscall per new chunk in write_blob_bytes"

This reverts ee51b32. The create_new change showed no measurable throughput
benefit — three 9-rep interleaved runs on the same ext4 device swung −12%..+21%
at the 256 KiB CDC size (a negative stat on a warm dentry cache is ~µs, below
the shared-disk noise floor). Applying the same "no change without a measured
win" bar used for the pool-sizing revert: the idiom/TOCTOU angle is real but the
race is already prevented upstream by the PG pin-or-classify serialisation, so
it's defence-in-depth only — not enough to keep an unmeasured change. Reverts
the production change, the bench, and its doc together.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JG5yYZ9s868mJwqT2Qz7ez

src/infrastructure/services/local_blob_backend.rs

@@ -128,29 +128,12 @@ async fn fsync_paths_parallel(paths: Vec<PathBuf>, strict: bool) -> Result<(), D
 /// (fsync now vs. deferred batch sync), or `None` when the blob already
 /// existed (idempotent skip — content-addressed, so identical by definition).
 async fn write_blob_bytes(blob_path: &Path, data: &Bytes) -> Result<Option<File>, DomainError> {
-    // `create_new` = `open(O_CREAT | O_EXCL)`: ONE syscall that both tests for
-    // existence and creates, replacing the old `try_exists` (stat) + `File::create`
-    // (two metadata syscalls, each its own `spawn_blocking` round-trip on Tokio's
-    // blocking pool). On a large upload of unique data that's thousands of stats
-    // saved. The blob is content-addressed, so an already-present file is
-    // byte-identical by definition → `AlreadyExists` is exactly the idempotent
-    // skip the old `try_exists` branch performed — and O_EXCL closes the TOCTOU
-    // window the check-then-create pair left open (no truncate-over-a-racing-writer).
-    let mut file = match fs::OpenOptions::new()
-        .write(true)
-        .create_new(true)
-        .open(blob_path)
-        .await
-    {
-        Ok(file) => file,
-        Err(e) if e.kind() == std::io::ErrorKind::AlreadyExists => return Ok(None),
-        Err(e) => {
-            return Err(DomainError::internal_error(
-                "Blob",
-                format!("Failed to create blob file: {}", e),
-            ));
-        }
-    };
+    if fs::try_exists(blob_path).await.unwrap_or(false) {
+        return Ok(None);
+    }
+    let mut file = fs::File::create(blob_path).await.map_err(|e| {
+        DomainError::internal_error("Blob", format!("Failed to create blob file: {}", e))
+    })?;
     file.write_all(data).await.map_err(|e| {
         DomainError::internal_error("Blob", format!("Failed to write blob from bytes: {}", e))
     })?;