perf: round 6 backend — CardDAV cursor streaming, borrowed NC id chain, binary UUID decode, one-alloc hex

Benchmark-gated (equivalence + BEFORE/AFTER in examples/bench_*, results
and reproduce commands in benches/ROUND6.md):

- CardDAV whole-book REPORT + depth-1 PROPFIND stream through a PG
  cursor (stream_contacts_by_book, 500-contact pages) instead of
  materialising every vCard twice: 8 000 contacts TTFB 37.4 → 7.6 ms
  (4.9x), peak heap 19.0 → 7.0 MiB (2.7x), wall -23%; REPORT and
  PROPFIND byte-identical to the buffered writers.

- NC numeric-id chain fully borrowed: get_or_create_file_ids/folder_ids
  take &[&str] and return HashMap<Uuid, i64>; batch_resolve_ids callers
  (PROPFIND pages, REPORT, trashbin, OCS search) pass id slices and look
  up via nc_id_of. 2.006 → 0.006 allocs/child (334x), 1.53x wall per
  500-child page. batch_check_favorites binds &[&str] as text[].

- file_blob_read_repository listing SELECTs drop id::text/folder_id::text
  server casts: rows decode binary Uuid (16 vs 36 bytes on the wire) and
  render once in row_to_file. A/B on 500-row pages: 1.225 → 1.044 ms
  mean (1.17x), p95 1.686 → 1.345 (bench_uuid_text_cast; single-row,
  param and min() sites left as-is deliberately).

- IncrementalHasher::finalize_hex renders through common::fmt::hex_lower
  instead of one format! per digest byte: 18 → 1 (md5) / 35 → 1 (sha256)
  allocs per chunk finalize, 14-15x wall.

- Share landing overlaps the access-count UPDATE with the unlock fetch
  via tokio::join! (one round-trip off every public link hit).

- REJECTED by benchmark and reverted: try_join_all fan-out of the
  batch-favorites authz pre-check — 42.6 → 56.4 ms cold, 0.15 → 0.23 ms
  warm against local-socket PG (bench_favorites_authz kept as evidence).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017aJu9ghvuT8WqC31ZEGTBA

src/interfaces/api/handlers/share_handler.rs

@@ -232,17 +232,18 @@ pub async fn access_shared_item(
     Path(token): Path<String>,
     headers: HeaderMap,
 ) -> impl IntoResponse {
-    // Register the access
-    let _ = share_use_case.register_shared_link_access(&token).await;
-
     // Honour an unlock cookie if one was issued by a prior `/verify` call.
     let unlock_jwt = unlock_jwt_from_headers(&headers, &token);
 
-    // Get the shared link
-    match share_use_case
-        .get_shared_link_with_unlock(&token, unlock_jwt.as_deref())
-        .await
-    {
+    // The access-count increment doesn't gate the fetch — run both
+    // round-trips concurrently instead of serially (one RTT saved on
+    // every public share landing).
+    let (_, item) = tokio::join!(
+        share_use_case.register_shared_link_access(&token),
+        share_use_case.get_shared_link_with_unlock(&token, unlock_jwt.as_deref()),
+    );
+
+    match item {
         Ok(item) => (StatusCode::OK, Json(item)).into_response(),
         Err(err) => {
             // Special handling for share access errors