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_31.txt

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