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_25.txt

@@ -0,0 +1,17 @@
+                                      
+             dtopcttttd               
+           tgptpedcoepeet             
+          5e55ttg-etoooeeed           
+         etpe 5g oe goetpo5           
+        tddot5pdc5deg e55o5p          
+        otxexdpt-dec 5ete55et         
+        e5epe edd5od5eo5dgeoe         
+        eee5e-ggxdoo5eodxoeeo         
+       dtoegeddooootxeooetpeo         
+        5-ge5etedeecpdeopo5oe         
+        oxp5oeegggggpt5eoe5ee         
+         edgectco-tpcd5t55e5          
+          dededodpc5td5dee5           
+           o-coodpoeppgpep            
+             xtgpottdtep              
+