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_35.txt

@@ -0,0 +1,17 @@
+                                      
+              dttcotttottd            
+           tpop-xpptgepxcgxpod        
+        d55ectpedpge  egpop-gept      
+       t55t5tctd            ptdoed    
+      55xe55o gopt            eopet   
+     c5c5te pedg--pd           eooe   
+     o g-5    oo ppd-           edoo  
+     xexc      5ooc5p           5 g5  
+     5 td    tee5cg5eoodcdotxx  exop  
+     etdcp  55cgpt5ec otdddd5gx5p ep  
+      5eotp5ode5de  egddddpddp55dte   
+       g-do5x               d5p td    
+        ocedctct          tc5dppp     
+          gddgxpx-cxxtxc5pgd5cg       
+             gdxcodd5ddttpgd          
+