Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/tui_app_server/frames/shapes/frame_34.txt

@@ -0,0 +1,17 @@
+                                      
+               ◆▲●□□□□●◇▲◆            
+           ◆□□□△○●◇■◇□□●□□●▲●◆        
+         ◆△△▲◇□■△◇□◆   ◆○○○▲■□□▲      
+        ▲△◆◇□◆△▲◆          □◇▲□○○     
+       △△◇○▲△◇ ◇■ ▲          ○○□○○    
+      ▲△○◇◆△ ○○■◇○□○          ○▲○◇○   
+      ◇□○◇◆   ■○◆□◇◆○          ◇●◇◆   
+      ○◇●■△    ◇□◆◇△■◇         △ ◆○   
+      ▲■△△■   △○△△■◆●●□□□□□□◇◆◆■ ◇○◆  
+      ◇◇○▲△▲▲○■△△■◇◇◇◆◇△◆◆◆▲△◆◇◇◆◇○   
+       △◆○▲ ◇○◇□○   ■□□◆◆◆◆○ ▲△▲○◇■   
+        △▲○●●○◆             △●△◇△■    
+         ○□◆◇□○●▲        ◆□●■◆△▲◆     
+           ▲○△◇■◇□▲●▲□●□●□◆□□△■       
+             ■◇○●●◆◆◆◆◆●●□●○          
+                   ◆◆◆