docs(engine): evaluate multi-root transfer scenarios (#1690)

[oferchen]

Closes #1690.

Summary

• Adds docs/audits/multi-root-transfer-scenarios.md evaluating whether multi-root rsync invocations (oc-rsync src1/ src2/ src3/ dst/) require a multi-producer WorkQueue.
• Traces the call path (extract_operands -> LocalCopyPlan::from_operands -> copy_sources -> sender's build_file_list) and confirms oc-rsync mirrors upstream rsync 3.4.1 byte-for-byte: serial per-arg traversal, single merged-sort, single ordered NDX stream (flist.c:2258-2271, generator.c:2312-2315).
• Recommends closing trackers Fix stats tests by clearing skip-compress env #1382, Align brand fallback with workspace metadata #1405, Fix Windows device identifier metadata usage #1610, Clarify --bwlimit burst syntax in help output #1383 as not-required: the wire protocol's MonotonicNdxWriter invariant and the receiver's single multiplex stream make multi-producer dispatch architecturally redundant. Keep Expand documentation branding validation coverage #1572/Fix Windows cross-compilation by gating unix-only user lookups #1613 (benchmarks) and Align toolchain floor with CI checks #1404 (done) on engine-internal merit only.

Verification

• Docs-only PR. No .rs or Cargo.toml changes.
• No code paths exercised; CI's docs/markdown checks cover the rest.