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_9.txt

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