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_17.txt

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