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_34.txt

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