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_3.txt

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