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_32.txt

@@ -0,0 +1,17 @@
+                                      
+                tttttccxtd            
+              cpxppxoeeppext          
+            5dpodttdc-epepoppt        
+           p55edtec- - tdoettgt       
+          55etepoppec petxo5opot      
+          o5ootopeeceetd   et5 p      
+         pegctepoeettetoe5d55epd      
+         eeetd  gdeeg5pec-dgoegge     
+         oee-d pdegddetttxxxoegoe     
+         pggdeepopodddddeepecx-d      
+           topee5 epggpppdc555-5      
+           otedoo        toe5px       
+           pppeextd    d5de5 t        
+             o-ogxtecopedtgd5         
+               xcoddtt5pdgg           
+                   eee