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_4.txt

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