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_8.txt

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