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_25.txt

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