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_36.txt

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