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_24.txt

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