Implement SSH X11 forwarding Phase 4: Save X11ForwardContext

- Save X11ForwardContext to Channel.x11_forward_context
- Clone context for later use in data forwarding
- Prepare for actual X11 data forwarding in handle_channel_data

All 182 tests pass.

markbase-core/src/ssh_server/channel.rs

@@ -1517,6 +1517,11 @@ direct_tcpip: None,
         let display = std::env::var("DISPLAY").unwrap_or_else(|_| ":0".to_string());
         let x11_ctx = super::x11_forward::X11ForwardContext::new(&display)?;
         
+        // Phase 4: 保存 X11ForwardContext 到 Channel
+        if let Some(ch) = self.channels.get_mut(&channel) {
+            ch.x11_forward_context = Some(x11_ctx.clone());
+        }
+        
         // 设置 DISPLAY 环境变量（client 会使用）
         // Server 需要在 exec/shell 环境中设置 DISPLAY
         // 这里只是记录，实际设置在 exec/shell handler 中