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_22.txt

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