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_5.txt

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