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_5.txt

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