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/application/services/favorites_service.rs

@@ -145,6 +145,12 @@ impl FavoritesUseCase for FavoritesService {
         // valid (partial success would leak the same oracle we
         // closed on the single-item path). See
         // `docs/plan/authz_audit/rest_storage.md`.
+        //
+        // Deliberately serial: a `try_join_all` fan-out measured WORSE
+        // on both the cold (drive_of point-SELECTs) and warm (all-moka)
+        // paths — future orchestration + pool-acquire contention cost
+        // more than the local round trips they overlap. Rejected by
+        // `bench_favorites_authz`; numbers in benches/ROUND6.md.
         for (item_id, item_type) in items {
             let resource = Resource::parse(item_type, item_id)?;
             self.authorization