perf(round28): extend the PROPFIND oc:id reused buffer (round27 H1) to the REPORT emit loops

report_handler's four REPORT emit loops (the filter-files favorites REPORT and the
search REPORT, each a file loop + a folder loop) shared the same per-row oc:id
String that ROUND27 §H1 replaced in the two PROPFIND page loops. Apply the
identical, already-validated transformation: hoist one oc_buf per handler (reused
across both its loops) and compute the id into it via format_oc_id_into instead of
a fresh format_oc_id String per child. 1 String/row -> 0 (amortized). The
write_{file,folder}_response fns already take Option<&str>, so their signatures are
unchanged and the emitted oc:id bytes are byte-identical.

Same transformation benchmarked in ROUND27 §H1 (bench_round27_micro: 998 -> 0
per-row allocs, 2.16x wall), so no new bench. Verified: cargo fmt clean, cargo
clippy --features bench -D warnings clean, cargo test --lib --features bench =
529 passed / 0 failed across 5 consecutive runs.

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

benches/ROUND28.md

@@ -0,0 +1,75 @@
+# Round 28 — extend the PROPFIND oc:id buffer (ROUND27 §H1) to the REPORT emit loops
+
+A small follow-through: ROUND27 §H1 replaced the per-row `oc:id` `String` with one
+reused `oc_buf` in the two NextCloud **PROPFIND** page loops, but the four
+**REPORT** emit loops (`report_handler`) shared the identical per-row-String
+shape and were explicitly deferred there. This round applies the same validated
+transformation to them.
+
+## The change
+
+`report_handler`'s two REPORT handlers (`filter-files` favorites REPORT and
+`search` REPORT) each emit a file loop and a folder loop, and each row did:
+
+```rust
+let oc_id = fid.map(|id| format_oc_id(id, file_id_svc));   // one String per row
+…
+write_{file,folder}_response(&mut xml, …, (fid, oc_id.as_deref()), …)
+```
+
+AFTER hoists one `oc_buf` per handler (reused across both its loops, beside the
+same pattern the PROPFIND loops already use) and computes the id into it with
+`format_oc_id_into` (added in ROUND27):
+
+```rust
+let mut oc_buf = String::new();          // once per handler
+…
+let oc_id: Option<&str> = match fid {
+    Some(id) => { format_oc_id_into(&mut oc_buf, id, file_id_svc); Some(oc_buf.as_str()) }
+    None => None,
+};
+write_{file,folder}_response(&mut xml, …, (fid, oc_id), …)
+```
+
+**1 String/row → 0** (amortized to one buffer per handler) across all four REPORT
+loops. The `write_*_response` functions already take `Option<&str>`, so their
+signatures are unchanged and the emitted `oc:id` bytes are byte-identical.
+
+## Benchmark
+
+This is the **same** transformation validated in ROUND27 §H1
+(`bench_round27_micro`): a per-row `format_oc_id` String vs one reused buffer via
+`format_oc_id_into`, byte-identical output. §H1 measured it on a 500-row page:
+
+| arm    |    ns/op | allocs/op |
+|--------|---------:|----------:|
+| BEFORE | 34 185.3 |  1 000.00 |
+| AFTER  | 14 484.9 |      2.00 |
+
+**998 → 0 per-row allocs, 2.16–2.36× wall.** ROUND28 applies that proven change
+to four more instances of the identical pattern (the REPORT loops), so no new
+benchmark is needed — the §H1 gate is the evidence. REPORT/search is lower-traffic
+than PROPFIND, so the aggregate impact is smaller, but it removes the last per-row
+`oc:id` allocation from the NC emit surface.
+
+## Not shipped — carried forward
+
+- **`format_oc_id_into` for the trashbin per-item writer** (`write_trash_item_response`)
+  would need the buffer threaded through its signature (it is a per-item fn, not a
+  loop with a hoisted buffer); low traffic, deferred.
+- **REPORT per-row `href` buffer** (`nc_href` allocates per row) — the ROUND20
+  deferred href-buffer item; wants an `nc_href_into` + a precomputed encoded-user,
+  a separate alloc pass.
+- **S3 read zero-copy forward** — a genuine framing tradeoff (fewer, larger
+  coalesced frames vs more, smaller zero-copy frames) that cannot be faithfully
+  benchmarked without a real S3/MinIO fixture; not shipped on synthetic evidence.
+- **Frontend folder-listing cache / `/resources` ETag** — the real bandwidth win
+  needs a backend ETag on the listing feed + conditional 304, plus SWR wiring that
+  respects cursor pagination. A dedicated backend+frontend feature.
+
+## Environment / methodology
+
+- Source-only extension of the ROUND27 §H1 change; the benchmark evidence is
+  `bench_round27_micro` §H1. Verified: `cargo fmt --all --check` clean,
+  `cargo clippy --features bench -- -D warnings` clean, `cargo test --lib
+  --features bench` green.