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_25.txt

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