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_36.txt

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+     #A-**█.*-*#.             -*.*    
+     A #A-  *- █.A#            █*-*   
+    . * -      * █**            .-.#  
+    ..*.      -* -AA            *. .  
+    .██.     AA #*██###-#####-  .*..  
+     -*.*  #.--A.A  -.-------.. *A█-  
+     █ *.* #-#A-     █**-----█ .#█A   
+      █#█**.                 #A.█A    
+        *A..#.#           ###█A#-     
+          *#-█***-*.-#.*█-█#*#█       
+            -*#A.-##-####**█          
+