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/dots/frame_26.txt

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+                                      
+              ○◉●●● ●●                
+             ◉●·◉◉○·●◉·●              
+            ◉●  ·◉○·○  ○●             
+           ◉● ●···○··○●◉○             
+           ·○●○·◉· ●○◉◉·○◉            
+           ○  ·◉◉◉ ●○○ ○○◉            
+          ·   ●●○··◉··◉◉ ●            
+          ◉◉  ○···○○○··◉··            
+          ·  ◉○●◉●·●○○◉○··            
+            ◉○·◉●·○○●●○○●·            
+           ·· ◉·● ○●◉·●○··            
+           ○○··◉○   ◉◉ ···            
+            ●  ●·○ ○·◉·◉●             
+             · ●○○ ●○●●◉              
+              ○◉◉○●·○○●               
+