Add feasibility decision memo for Tauri 2.0 + mobile migration proposal

[Copilot]

This PR adds a structured technical assessment for the proposal to migrate DevSidecar from Electron to Tauri 2.0 and extend support to Android/iOS. It focuses on feasibility, scope realism, and a go/no-go decision framework for implementation planning.

• Decision summary

  • Recommends accepting the strategic direction (Tauri + Rust core + mobile support).
  • Rejects the current assumption of a one-shot full migration in 2–3 weeks.

• Architecture feasibility analysis

  • Evaluates current coupling in Electron main process and Node-based MITM/runtime paths.
  • Clarifies that Rust MITM adoption is a reimplementation effort, not a lift-and-shift.
  • Positions EasyTier as a potential network-layer complement, not a MITM replacement.

• Re-baselined delivery estimate

  • Reframes delivery into phased milestones:
    • A: core decoupling + Rust MITM POC
    • B: desktop Tauri parity
    • C: mobile MVP
  • Updates overall estimate to a staged 7–14 week window.

• Execution gates

  • Adds explicit continuation criteria (POC stability, rollback path, scoped mobile MVP, iOS signing/compliance readiness) to support objective acceptance decisions.

Example excerpt from the new memo:

- **接受方向**：支持探索 Tauri 2.0 + Rust 内核，目标覆盖移动端。
- **不接受当前“一次性整体迁移、2-3 周完成”的实施假设**。
...
> 合计建议：**7-14 周**（按阶段验收，可随阶段成果提前止损或继续）。