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_31.txt

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