docs(transfer): document ReorderBuffer head-of-line blocking semantics (#1883)

[oferchen]

Summary

Adds docs/architecture/reorder-buffer.md documenting the head-of-line
(HoL) blocking behaviour of the concurrent delta pipeline's reorder buffer
and contrasting it against upstream rsync 3.4.1's strictly-sequential
per-file pipeline.

Tracker: oc-rsync task #1883.

The doc covers:

• Both reorder-buffer implementations and where they sit in the pipeline:
  the ring-buffer ReorderBuffer (crates/engine/src/concurrent_delta/reorder.rs:65)
  owned by the delta-reorder consumer thread
  (crates/engine/src/concurrent_delta/consumer.rs:147-188), and the
  alternative sliding-window BoundedReorderBuffer
  (crates/transfer/src/reorder_buffer.rs:57) with its explicit
  BackpressureError.
• Current HoL semantics: a slow file at next_expected stalls every
  completed successor in the bounded window (default
  2 * rayon::current_num_threads() slots), and force_insert
  (reorder.rs:334) trades the memory bound for forward progress when
  the head slot is empty and the window fills.
• Trade-offs vs upstream's recv_files loop
  (target/interop/upstream-src/rsync-3.4.1/receiver.c:522): higher
  throughput under uniform file sizes, but a "stop-then-burst" UX pattern
  on skewed distributions.
• When HoL blocking is observable: mixed large-and-small workloads,
  --delay-updates runs (mirroring upstream's handle_delayed_updates at
  receiver.c:422), phase 2 redo, and the small-file regime where it is
  effectively invisible.
• Pointers to related work-in-progress trackers: Ensure release workflow uploads architecture-specific assets #1884 (bounded-memory
  spill-to-tempfile for stalled successors), Ensure release task uploads cross-compiled binaries #1885 (stall-duration metrics),
  and Ensure release checksums are verifiable #1886 (bypass parallel pipeline when --delay-updates is off). All
  three are described as open design items, not merged code.

Documentation only - no code changes, no other docs cross-linked.

Test plan

• git diff confirms only docs/architecture/reorder-buffer.md is added.
• CI: fmt+clippy, nextest stable, Windows stable, macOS stable, Linux musl
  stable, interop. Docs-only PR; no code paths touched.