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_34.txt

@@ -0,0 +1,17 @@
+                                      
+               dtottttoxtd            
+           dtpt5ocegxtpoppctod        
+         d55tepgcxpe   edootgppt      
+        t5depd5td          petpoo     
+       55eo-5egeggt          oopod    
+      t5oee5 oogeopo          otoeo   
+      epdxd   podpedd          ecxd   
+      oeogc    epde5ge         5 do   
+      tp5-g   5o55gdoottttttxddg xde  
+      eeot5ttdg55pxeedx-dddt5deeeeo   
+       5dot eoepdg  gppeeeed t5toep   
+        5tocood             5c-e5p    
+         oteetoct        dtogd5te     
+           -o5xgettcttopopetpcp       
+             gxocodddddcopod          
+                   eee