docs(design): async migration plan with incremental adoption (#1594)

[oferchen]

Summary

• Replaces the prior 762-line draft of docs/design/async-migration-plan.md with an 82-line roadmap (under the 100-line cap).
• Documents the current async state (tokio in daemon and core), pressure points (async listener, async SSH, io_uring socket I/O), and a four-phase incremental strategy.
• Records anti-goals (no async per-file fanout, no async file syscalls) and feature-gating compat rules.

Test plan

• Markdown renders cleanly on GitHub
• Cross-references (CI: remove unsupported windows aarch64 target #1593, Switch flate2 to the Rust backend for cross builds #1595, Add zlib accessor coverage to boost test totals #1732, Refactor local copy options into dedicated module #1751, Fix Windows PATHEXT parsing for fallback binary search #1818, Accept quoted fallback override values #1934, Normalize quoted fallback keywords #1935) resolve to existing issues