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_2.txt

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