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_18.txt

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