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_33.txt

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