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_22.txt

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