perf(round27): NextCloud PROPFIND oc:id per-row buffer, contact JSONB write direct-serialize

Two behaviour-preserving allocation cuts (benches/ROUND27.md), each with a
counting-allocator BEFORE/AFTER gate that rolls back if AFTER does not allocate
fewer than BEFORE:

- H1 NextCloud PROPFIND: the streaming page loops built oc:id as a fresh String
  per child (format_oc_id -> format!("{:08}{}", id, instance)). Add
  format_oc_id_into(&mut buf, id, svc) and compute into one oc_buf reused across
  the page (next to the existing href buffer) — 1 String/row -> 0. 998->0
  per-row allocs on a 500-row page, 2.16x wall. The write fns still take
  Option<&str>, so no signature change; oc:id bytes identical. Scoped to the two
  PROPFIND page loops (the hot directory-listing path); REPORT/trashbin deferred.
- P2 contact create/update: bind sqlx::types::Json(&dtos) (Encode runs to_writer
  straight into the JSONB buffer) instead of serde_json::to_value(&dtos) + bind,
  which built a throwaway Value DOM per JSONB column. Write-side twin of ROUND23
  J1. 21->2 allocs, 4.68x wall for a 3-entry column. Behaviour-preserving:
  to_value sorts keys and direct serialize keeps struct order, but Postgres
  normalizes JSONB key order so the stored value is identical (verified via psql:
  '{...alpha...}'::jsonb = '{...struct...}'::jsonb -> t), and reads decode by
  field name; the etag comes from the domain entity, not the stored JSONB.

Adds bench_round27_micro. Verified: cargo fmt clean, cargo clippy --features
bench -D warnings clean (real exit), cargo test --lib --features bench = 529
passed / 0 failed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L8gs91AhmazoxMsDcNk3KT

Cargo.toml

@@ -358,6 +358,17 @@ name = "bench_micro_allocs"
 path = "examples/bench_micro_allocs.rs"
 required-features = ["bench"]
 
+# Round-27 battery ────────────────────────────────────────────────────────────
+
+# Round-27 CPU/alloc micro-pack (no Postgres) — NC PROPFIND per-row oc:id String
+# → reused buffer via format_oc_id_into (H1); contact create/update JSONB write
+# through a throwaway serde_json::Value DOM → sqlx::types::Json(&dtos) direct
+# serialize (P2, the write-side twin of §J1).
+[[example]]
+name = "bench_round27_micro"
+path = "examples/bench_round27_micro.rs"
+required-features = ["bench"]
+
 # Round-26 battery ────────────────────────────────────────────────────────────
 
 # Round-26 CPU/alloc micro-pack (no Postgres) — drive-policy JSONB decode through