docs(design): async migration plan for transfer pipeline

[oferchen]

Summary

• Refresh the async migration roadmap to align with the
  seven-crate tokio surface ratified by the boundary
  re-verification audit (PR docs(audits): re-verify tokio dependency boundary #3706,
  docs/audits/tokio-dependency-boundary-2026.md).
• Replace the earlier 5-phase sketch with a 6-phase plan: Phase 0
  wires a CI guardrail; Phase 1 ratifies the seven crates;
  Phases 2-4 promote feature-gated async paths to default for the
  daemon listener, SSH transport, and receiver pipeline; Phase 5
  finalises the rayon-tokio composition.
• Each phase carries a build-time feature flag and a runtime kill
  switch with four exit gates per rollout (wire compatibility,
  performance, coverage, interop).
• Cites the daemon thread-per-connection scalability audit
  (docs(audits): daemon thread-per-connection scalability audit (#1673) #3705), the channel-overhead audit (Fix doc links in transport negotiation docs #1592), the SSH async
  evaluation audit (CI: remove unsupported windows aarch64 target #1593), and the io_uring-async interaction
  audit (Switch flate2 to the Rust backend for cross builds #1595).

This supersedes #3664 (the earlier 5-phase sketch) by aligning
the migration narrative with the post-audit reality. Branch
opened as -v2 because the original docs/async-migration-plan-1594
branch was consumed by the merged PR.

Test plan

• Doc renders cleanly under existing markdown lint.
• grep -ni -E 'claude|internal|internal docs\.md' docs/design/async-migration-plan.md empty.
• grep -nE '—|–' docs/design/async-migration-plan.md empty.
• All file:LINE citations resolve in the current tree.
• No code changes; CI lint and clippy unchanged.