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_12.txt

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