feat(tui2): add feature-flagged tui2 frontend (#7793)

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.

<img width="619" height="364" alt="image"
src="https://github.com/user-attachments/assets/4893f030-932f-471e-a443-63fe6b5d8ed9"
/>

codex-rs/Cargo.toml

@@ -34,6 +34,7 @@ members = [
     "stdio-to-uds",
     "otel",
     "tui",
+    "tui2",
     "utils/git",
     "utils/cache",
     "utils/image",
@@ -88,6 +89,7 @@ codex-responses-api-proxy = { path = "responses-api-proxy" }
 codex-rmcp-client = { path = "rmcp-client" }
 codex-stdio-to-uds = { path = "stdio-to-uds" }
 codex-tui = { path = "tui" }
+codex-tui2 = { path = "tui2" }
 codex-utils-cache = { path = "utils/cache" }
 codex-utils-image = { path = "utils/image" }
 codex-utils-json-to-toml = { path = "utils/json-to-toml" }