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_30.txt

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