fix: 隧道稳定性——假死强制重连/重试预算重置/stderr 实时读取

[modusensus]

• daemon: 连续 3 次健康检查失败且 SSH 进程仍存活（假死）时强制 stop() 触发重连，不再留隧道永久断线
• retry: 会话稳定运行 >= stable_after(60s) 后重置重试预算，长跑隧道不因前期抖动永久放弃
• health: 检查失败指数退避（60s→…→300s 封顶），避免高频新开 SSH 触发服务器 MaxStartups
• core: stderr 独立线程实时读取并记录断线原因，不再等进程退出才可见；新增 last_session_duration 供 retry 层判定稳定会话
• 版本 bump 0.2.0 → 0.2.1，109 测试全绿

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