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_7.txt

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