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_20.txt

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