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_32.txt

@@ -0,0 +1,17 @@
+                                      
+                ●●●●●◉◉·●○            
+              ◉●·●●·○··●●··●          
+            ◉○●○○●●○◉◉·●·●○●●●        
+           ●◉◉·○●·◉◉ ◉ ●○○·●● ●       
+          ◉◉·●·●○●●·◉ ●·●·○◉○●○●      
+          ○◉○○●○●··◉··●○   ·●◉ ●      
+         ●· ◉●·●○··●●·●○·◉○◉◉·●○      
+         ···●○   ○·· ◉●·◉◉○ ○·  ·     
+         ○··◉○ ●○· ○○·●●●···○· ○·     
+         ●  ○··●○●○○○○○○··●·◉·◉○      
+           ●○●··◉ ·●  ●●●○◉◉◉◉◉◉      
+           ○●·○○○        ●○·◉●·       
+           ●●●···●○    ○◉○·◉ ●        
+             ○◉○ ·●·◉○●·○● ○◉         
+               ·◉○○○●●◉●○             
+                   ···