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_17.txt

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