refactor: 重构 starssh 核心运行时并补强 ssh/exec/terminal/sftp 能力

- 拆分原有单体 ssh.go，按职责重组为 types、utils、transport、login、keepalive、session、exec、pool、shell、terminal、forward、hostkey、state 等模块，并补充平台相关实现
  - 重做登录与连接运行时，补齐基于 context 的建连、jump/proxy 链路、可配置认证顺序，以及 Unix/Windows 下的 ssh-agent 支持
  - 新增正式非交互执行模型 ExecRequest/ExecResult，支持流式输出、溢出统计、超时控制，以及 posix/powershell/cmd/raw 多方言执行
  - 保留旧 shell 风格兼容接口，同时让路径/用户探测等 helper 具备跨 shell fallback，避免 Windows 目标继续硬依赖 POSIX 命令
  - 新增 TerminalSession 作为原始交互终端基座，提供 IO attach、resize、signal/control、退出状态与关闭原因管理
  - 重构端口转发语义，默认复用当前 SSH 连接，并显式提供 detached 的本地/动态转发模式承载隔离场景
  - 梳理 keepalive 与取消语义，区分仅取消本次操作和关闭整条连接，并统一连接状态与传输关闭路径
  - 围绕新的 session/连接生命周期重做执行池与运行时支撑
  - 大幅增强 SFTP 传输链路，补齐更安全的原子替换、校验、进度回调、重试隔离、可复用 client 生命周期与失败语义
  - 新增取消语义、keepalive、SFTP、forward、terminal input 等关键回归测试，提升核心链路稳定性

state.go

@@ -0,0 +1,106 @@
+package starssh
+
+import (
+	"errors"
+
+	"golang.org/x/crypto/ssh"
+)
+
+type sshClientRequester interface {
+	SendRequest(name string, wantReply bool, payload []byte) (bool, []byte, error)
+	Close() error
+}
+
+var closeSSHClient = func(client sshClientRequester) error {
+	if client == nil {
+		return nil
+	}
+	return client.Close()
+}
+
+func (s *StarSSH) snapshotSSHClient() *ssh.Client {
+	if s == nil {
+		return nil
+	}
+
+	s.stateMu.RLock()
+	defer s.stateMu.RUnlock()
+	return s.Client
+}
+
+func (s *StarSSH) requireSSHClient() (*ssh.Client, error) {
+	client := s.snapshotSSHClient()
+	if client == nil {
+		return nil, errors.New("ssh client is nil")
+	}
+	return client, nil
+}
+
+func (s *StarSSH) setTransport(client *ssh.Client, upstream *StarSSH) {
+	if s == nil {
+		return
+	}
+
+	s.stateMu.Lock()
+	defer s.stateMu.Unlock()
+	s.Client = client
+	s.upstream = upstream
+	s.online = client != nil
+}
+
+func (s *StarSSH) detachTransport() (*ssh.Client, *StarSSH) {
+	if s == nil {
+		return nil, nil
+	}
+
+	s.stateMu.Lock()
+	defer s.stateMu.Unlock()
+
+	client := s.Client
+	upstream := s.upstream
+	s.Client = nil
+	s.upstream = nil
+	s.online = false
+	return client, upstream
+}
+
+func (s *StarSSH) takeKeepaliveHandles() (chan struct{}, chan struct{}) {
+	if s == nil {
+		return nil, nil
+	}
+
+	s.keepaliveMu.Lock()
+	defer s.keepaliveMu.Unlock()
+
+	stop := s.keepaliveStop
+	done := s.keepaliveDone
+	s.keepaliveStop = nil
+	s.keepaliveDone = nil
+	return stop, done
+}
+
+func (s *StarSSH) closeTransport(waitKeepalive bool) error {
+	if s == nil {
+		return nil
+	}
+
+	_ = s.closeReusableSFTPClient()
+
+	client, upstream := s.detachTransport()
+	stop, done := s.takeKeepaliveHandles()
+	if stop != nil {
+		close(stop)
+	}
+
+	var closeErr error
+	if client != nil {
+		closeErr = normalizeAlreadyClosedError(closeSSHClient(client))
+	}
+	if waitKeepalive && done != nil {
+		<-done
+	}
+	if upstreamErr := closeUpstream(upstream); closeErr == nil {
+		closeErr = upstreamErr
+	}
+	return closeErr
+}