feat(tui2): copy tui crate and normalize snapshots (#7833)

Introduce a full codex-tui source snapshot under the new codex-tui2
crate so viewport work can be replayed in isolation.

This change copies the entire codex-rs/tui/src tree into
codex-rs/tui2/src in one atomic step, rather than piecemeal, to keep
future diffs vs the original viewport bookmark easy to reason about.

The goal is for codex-tui2 to render identically to the existing TUI
behind the `features.tui2` flag while we gradually port the
viewport/history commits from the joshka/viewport bookmark onto this
forked tree.

While on this baseline change, we also ran the codex-tui2 snapshot test
suite and accepted all insta snapshots for the new crate, so the
snapshot files now use the codex-tui2 naming scheme and encode the
unmodified legacy TUI behavior. This keeps later viewport commits
focused on intentional behavior changes (and their snapshots) rather
than on mechanical snapshot renames.

codex-rs/tui2/frames/slug/frame_1.txt

@@ -0,0 +1,17 @@
+                                      
+             d-dcottoottd             
+         dot5pot5tooeeod dgtd         
+       tepetppgde   egpegxoxeet       
+      cpdoppttd            5pecet     
+     odc5pdeoeoo            g-eoot    
+    xp te  ep5ceet           p-oeet   
+   tdg-p    poep5ged          g e5e   
+   eedee     t55ecep            gee   
+   eoxpe    ceedoeg-xttttttdtt og e   
+    dxcp  dcte 5p egeddd-cttte5t5te   
+    oddgd dot-5e   edpppp dpg5tcd5    
+     pdt gt e              tp5pde     
+       doteotd          dodtedtg      
+         dptodgptccocc-optdtep        
+           epgpexxdddtdctpg           
+