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_4.txt

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