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_21.txt

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