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_34.txt

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