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/slug/frame_6.txt

@@ -0,0 +1,17 @@
+                                      
+             d-occtoottdd             
+          tdc5peoptettcdtptd          
+        ttteeepg   egx-optp5od        
+       5tepepdot        oteeeet       
+      poeeepootpo         -ooeot      
+     c-5e5edtceodet        -oo5e      
+     d txe   gdoe5do        dtede     
+     x exe   deox5ee        xtoee     
+     d-e-e  5peepc5tdxtxddttd-o5e     
+     eeot5dddct e5opteetcoooeteog     
+      5 e 5xeec5g  gpgggppgeoo5e      
+       t c 5te           tcd55ee      
+        -eodppt       dtdd5ptt        
+         egxptgtgcxddttppgccp         
+            d-cpttdddedttp            
+