perf: round 20 — iCal/vCard parse allocs, owned-DTO moves, Result-collect pre-size, NC etag/favorites emit

Benchmark-gated (benches/ROUND20.md), same rule as rounds 2-19: every change
ships with a BEFORE/AFTER counting-allocator micro-benchmark and a byte-value
equivalence gate; a non-winning AFTER is rolled back (never applied). The
rollback rule is encoded in the harness (GATE FAIL exit). All 8 sections pass.

Reproduce: cargo run --release --features bench --example bench_round20_micro

- A1 CalendarEvent iCal parse: replace the throwaway per-property
  HashMap<String,Vec<String>> (DTSTART/DTEND/RECURRENCE-ID) with a direct
  VALUE=DATE scan; prop_with_params kept #[cfg(test)] (6->2 allocs/event, 4.2x)
- A2 UserDto::from: add User::into_parts and MOVE image (<=512 KiB data URI)
  + ui_preferences JSON instead of cloning on every /api/auth/me (27->14 allocs)
- A3 parse_vcard: drop the per-line to_ascii_uppercase copy + the lines Vec;
  promote ascii_ci_contains to common::text and share it (8->1 allocs/contact)
- A4 Calendar/AddressBook DTO: into_parts move incl. custom_properties map (18->10)
- I1 file-listing repos: collect::<Result<Vec>>() size-hints to 0 and grows from
  capacity 0; pre-size with Vec::with_capacity (8->1 container reallocs, 4 sites)
- I4 plaintext_stream: lazy emit iterator instead of eager Vec collect (43x wall)
- C1 NC write_etag_element: borrowed pre-escaped quote events, no owned quoted
  String/escape re-alloc; byte-identical output (3->0 allocs/PROPFIND row)
- C3 NC favorites REPORT: map.remove() move instead of get().clone() (~7 allocs/fav)

Deferred (documented in ROUND20.md): NC oc:id/trashbin buffer reuse, I1 sibling
CardDAV/CalDAV listing paths, Contact JSONB Json<Vec<_>> decode, dedup
settle_batch &str bind, and a fast DoS-resistant hasher for hot trusted-key maps
(needs a dependency decision).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JsJjcVX9RoN96DMa35Wqzd

src/infrastructure/services/encrypted_blob_backend.rs

@@ -140,13 +140,18 @@ where
 }
 
 /// Turn a decrypted payload into a stream of bounded, zero-copy slices.
+///
+/// The emit-slice iterator is handed to `stream::iter` lazily — the closure
+/// owns `data` (a refcounted `Bytes`), so each `slice` is produced on demand
+/// as the consumer polls, rather than eagerly `collect`ing a `Vec` of
+/// ⌈len/64 KiB⌉ slice handles up front (benches/ROUND20.md §I4).
 fn plaintext_stream(data: Bytes) -> BlobStream {
     let len = data.len();
-    let slices: Vec<Result<Bytes, std::io::Error>> = (0..len)
-        .step_by(PLAINTEXT_EMIT_SIZE)
-        .map(|off| Ok(data.slice(off..len.min(off + PLAINTEXT_EMIT_SIZE))))
-        .collect();
-    Box::pin(futures::stream::iter(slices))
+    Box::pin(futures::stream::iter(
+        (0..len)
+            .step_by(PLAINTEXT_EMIT_SIZE)
+            .map(move |off| Ok(data.slice(off..len.min(off + PLAINTEXT_EMIT_SIZE)))),
+    ))
 }
 
 impl BlobStorageBackend for EncryptedBlobBackend {