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.
Introduce a new codex-tui2 crate that re-exports the existing
interactive TUI surface and delegates run_main directly to codex-tui.
This keeps behavior identical while giving tui2 its own crate for future
viewport work.
Wire the codex CLI to select the frontend via the tui2 feature flag.
When the merged CLI overrides include features.tui2=true (e.g. via
--enable tui2), interactive runs are routed through
codex_tui2::run_main; otherwise they continue to use the original
codex_tui::run_main.
Register Feature::Tui2 in the core feature registry and add the tui2
crate and dependency entries so the new frontend builds alongside the
existing TUI.
This is a stub that only wires up the feature flag for this.
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