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_16.txt

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