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_16.txt

@@ -0,0 +1,17 @@
+                                      
+                dotgpptxd             
+               5deepeeoeoo            
+              e55go5ooee po           
+             555 cpdoeoeeppe          
+             eecd5ppoeep5eeo          
+            eeepep eoo ge x-e         
+            ooe 5eteeee5pgt e         
+            e5c eeee5eeegc ee         
+            ooexetx5deegpt  5         
+            pgddtooope-de tde         
+            eeetgg5poecgc-xee         
+             eep ee  e55t  5          
+             oep  e  5t5dte           
+              oexgdpx55tde            
+               pc-docddcp             
+