perf: round 9 — decorator PUT reactivation, session/search/dedup alloc purges, PROPFIND join!, folder-level cascade

Benchmark-gated round (benches/ROUND9.md): every change carries a
BEFORE/AFTER bench with equivalence/safety gates; verdicts below are from
the committed harnesses on 4 cores / local PG 16.

Backend:
- Blob decorators (Retry/Cached) now forward put_blob_from_bytes_unsynced
  + sync_blobs — the trait default had silently reinstated HEAD-before-PUT
  per chunk on decorated remote stacks, undoing ROUND3 §8. Full production
  stack: 500 probes -> 0, 1.9x wall at 10 ms RTT (bench_s3_put §3).
- NC PROPFIND per-page enrichment triple (favorites / oc:fileid / dead
  props) overlapped with tokio::join!: 2.07x local, 2.86x at 5 ms RTT
  (bench_nc_enrich_join, injected-latency decide-by-bench).
- Search enrichment consumes its DTOs and carries the interned Arc<str>
  display fields end-to-end (SearchFileResultDto type change, OpenAPI
  shape preserved): enrich_file 2.0x, 11.6 -> 2.2 allocs/row; the NC
  REPORT conversion stops re-running all three classifiers per row
  (bench_search_enrich).
- NC session Arc end-to-end: SharedNcSession extractor (8 -> 0 allocs),
  Arc<FolderDto> chroot cache (4 -> 0/hit), single shared Arc<CurrentUser>
  + lazy span render (11 -> 6/build) (bench_nc_session).
- Storage micro-pack: atomic create_new chunk writes (2.1x fresh),
  stream_chunks over the manifest Arc (4097 -> 0 allocs/read incl. the
  Range path), manifest single-flight (herd 64 -> 1 loads), hex_lower for
  chunk Content-MD5 (18 -> 1 allocs) (bench_storage_micro).
- OCS capabilities memoized into OnceLock<[Bytes;2]>: 237x, 102 -> 0
  allocs/poll, byte-identical (bench_capabilities_static).
- Drive::is_empty COUNT(*) sum -> EXISTS: 34.4x on a 100k-file drive
  (bench_drive_is_empty).
- favorites/recents row-map ROUND7 port: path/name/blob_hash moved,
  -2.75 allocs/row (bench_resource_row_map §2).
- Folder rows decode binary UUIDs (ROUND6 §10 port): 1.03-1.07x page
  fetch, honest verdict incl. one noise-band wash documented
  (bench_folder_uuid_decode).
- Authz: file cascade decision decomposed into memoized folder-level
  decision + direct-grant lookup (ROUND8 deferred item). Cold shared-album
  first view 592 -> 418 µs/thumb; warm path unchanged; safety gates incl.
  new direct-grant sibling isolation, revoke-flush re-verified, full
  integration authz suite green (bench_thumbnail_cascade_cache).

Frontend (vitest gates committed beside the code):
- resolveLabel/resolveRecipient O(directory) scan -> id-keyed Map: 13.9x
  (recipients.bench.test.ts).
- ResourceList selection-prune effect skips when nothing is selected
  (100 -> 0 Set builds per drain) and the photos timeline reads a
  listener-fed mobile flag instead of matchMedia per recompute
  (listDerives.bench.test.ts).

Verification: cargo fmt + clippy --all-features --all-targets -D warnings
clean; 524 unit + 554 integration (--cfg integration_tests) tests pass;
frontend npm run check clean with 293 vitest tests green.

Deferred with rationale in ROUND9.md: CalDAV authz-before-fetch reorder
(maintainer sign-off), per-page batched parent resolution, JWT-claims
Arc<str>, batch_operations signature widening.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XDc9VtXvskJ6dnMRraSndn

src/infrastructure/services/local_blob_backend.rs

@@ -128,18 +128,41 @@ 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> {
-    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))
-    })?;
+    // One atomic O_CREAT|O_EXCL open replaces the old stat-then-create pair:
+    // `AlreadyExists` IS the idempotent skip (content-addressed names mean an
+    // existing file has identical content), saving a syscall + a blocking-pool
+    // dispatch on every new chunk of every upload.
+    let mut file = match fs::File::options()
+        .write(true)
+        .create_new(true)
+        .open(blob_path)
+        .await
+    {
+        Ok(f) => f,
+        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),
+            ));
+        }
+    };
     file.write_all(data).await.map_err(|e| {
         DomainError::internal_error("Blob", format!("Failed to write blob from bytes: {}", e))
     })?;
     Ok(Some(file))
 }
 
+/// Bench-only public wrapper (feature = "bench") over the private chunk
+/// writer so `examples/bench_storage_micro.rs` can A/B the open strategy.
+#[cfg(feature = "bench")]
+pub async fn write_blob_bytes_for_bench(
+    blob_path: &Path,
+    data: &Bytes,
+) -> Result<Option<File>, DomainError> {
+    write_blob_bytes(blob_path, data).await
+}
+
 /// Compile-time lookup table for the 256 two-digit lowercase hex prefixes ("00"…"ff").
 static HEX_PREFIXES: [&str; 256] = [
     "00", "01", "02", "03", "04", "05", "06", "07", "08", "09", "0a", "0b", "0c", "0d", "0e", "0f",