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_2.txt

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