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_1.txt

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