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_19.txt

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