docs(design): multi-producer WorkQueue for multi-root transfers

[oferchen]

Summary

Design note for #1382 covering a multi-producer extension to the bounded
delta WorkQueue so multi-root transfers (oc-rsync foo/ bar/ baz/ dst/)
and parallel directory enumeration (#1573) can fan in to one shared queue.

Key sections

• Problem statement, citing the serial multi-root loop at
  crates/engine/src/local_copy/executor/sources/orchestration.rs:79
  and the SPMC contract at
  crates/engine/src/concurrent_delta/work_queue/bounded.rs:11-21.
• Three multi-producer designs (Clone, Arc-wrapped, producer pool)
  evaluated against today's crossbeam-backed channel.
• Recommended API: with_producers(n) returning a Vec<WorkQueueSender>
  that surfaces the producer-count contract in the type system.
• Ordering via per-producer sequence ranges, with the atomic-counter
  fallback already validated by the existing test
  multi_producer_requires_atomic_sequence_coordination.
• Backpressure, drain_parallel interaction (auto-close on last sender
  drop), migration plan for multi-root, --files-from, and future async
  producers, plus risks (deadlock, sequence collisions, fairness,
  adaptive policy).

Wire-compat invariants

Zero impact. WorkQueue is internal engine coordination only; no
protocol bytes change. The wire-protocol receive path keeps its
single-producer contract documented in
crates/engine/src/concurrent_delta/multi_producer_audit.rs:8-19.

Test plan

• Doc lint: 536 lines (within the 400-650 target).
• git diff origin/master..HEAD confirms only
  docs/design/multi-producer-workqueue.md is added.
• CI: docs-only change, no code surface modified.