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/hash/frame_35.txt

@@ -0,0 +1,17 @@
+                                      
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+        -AA.*#*.-*█-  -█***.█.*#      
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+      AA..AA* █**#            .**.#   
+     *A*A#- █.-█..*-           .**.   
+     *  .A    ** █*-.           .-**  
+     ...A      A***A█           A █A  
+     A #-    #..A*█A.**-*-*#..  ..**  
+     .#-*█  AA*█*#A.* *#----A█.A█ .█  
+      A-*#█A*-.A--  -█----*--*AA-#.   
+       █.-*A.               -A* #-    
+        **--*#A#          #*A-**█     
+          █--█.*.-A..#.*A*█-A*█       
+             █-.**--A--##*█-          
+