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_3.txt

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