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_4.txt

@@ -0,0 +1,17 @@
+                                      
+             d-octootottd             
+         d-gtpe5geoo5pceg5ptd         
+       tpoeeetpge   edxodpe5ood       
+      55eteg-tt          geppopo      
+     5e5deoeocdet          ogoepo     
+    5eede  pdee5po          p ooet    
+    goece   pppotget         t gce    
+     e-o     te5pdee           eee    
+    deged   ce5gd55txtttddddtt eep    
+    eo5ge t555tdeeooet-dtc5dce55ge    
+     eecpotooto5   gppppppppd-eeee    
+      teogede             tdc5ppp     
+       ootcdgtd        d-dtpce5       
+         xeeodppoccocttoptoepe        
+            pooxcxdddddc-pe           
+