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_23.txt

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