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_7.txt

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