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/dots/frame_35.txt

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+                                      
+              ○●●◉○●●●○●●○            
+           ●●○●◉·●●● ·●·◉ ·●○○        
+        ○◉◉·◉●●·○● ·  · ●○●◉ ·●●      
+       ●◉◉●◉●◉●○            ●●○○·○    
+      ◉◉··◉◉○  ○●●            ·○●·●   
+     ◉◉◉◉●· ●·○ ◉◉●○           ·○○·   
+     ○  ◉◉    ○○ ●●○◉           ·○○○  
+     ···◉      ◉○○◉◉●           ◉  ◉  
+     ◉ ●○    ●··◉◉ ◉·○○○◉○○●··  ··○●  
+     ·●○◉●  ◉◉◉ ●●◉·◉ ○●○○○○◉ ·◉● ·●  
+      ◉·○●●◉○○·◉○·  · ○○○○●○○●◉◉○●·   
+        ◉○○◉·               ○◉● ●○    
+        ○◉·○◉●◉●          ●◉◉○●●●     
+           ○○ ·●·◉◉··●·◉◉● ○◉◉        
+              ○·◉○○○◉○○●●● ○          
+