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.
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