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_6.txt

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