docs(design): tokio async listener implementation plan (#1935)

[oferchen]

Summary

• Adds docs/design/daemon-tokio-async-listener-impl.md (121 lines).
• Implementation plan for the Tokio-based daemon accept loop:
  • tokio::net::TcpListener accept loop, one task per connection.
  • Bridges to the existing sync transfer worker via spawn_blocking.
  • Gated behind a new async-daemon Cargo feature (off by default).
• Cross-references Accept quoted fallback override values #1934 (accepted RFC) and Refactor local copy options into dedicated module #1751 (async migration roadmap).

Closes #1935.

Test plan

• cargo fmt --all -- --check
• Markdown renders correctly on GitHub (no broken cross-references).
• No code changes; no compile/test impact.