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_36.txt

@@ -0,0 +1,17 @@
+                                      
+              ddtottttottd            
+          doggot5c5totcttgpptd        
+        topottp-pgee egpxptetpet      
+      degptdddd            ppxoge     
+     t5dcopeoeot-             do-p    
+     5 t5e  pd ge5t            godp   
+    e cge     go goo            edet  
+    eeox      do d55g           oe e  
+    epge     55 tpgptttdtttttd  eoxe  
+     dpeo  tedd5x5 gexdddddddee o5pe  
+     p peo tdt5d     gppdddddg etg5   
+      ptgoc-                 t5eg5    
+        o5eetxt           tttg5te     
+          ptdgppodcxdtxcg-gtctp       
+            ept5xdttdttttppg          
+