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/src/style.rs

@@ -0,0 +1,28 @@
+use crate::color::blend;
+use crate::color::is_light;
+use crate::terminal_palette::best_color;
+use crate::terminal_palette::default_bg;
+use ratatui::style::Color;
+use ratatui::style::Style;
+
+pub fn user_message_style() -> Style {
+    user_message_style_for(default_bg())
+}
+
+/// Returns the style for a user-authored message using the provided terminal background.
+pub fn user_message_style_for(terminal_bg: Option<(u8, u8, u8)>) -> Style {
+    match terminal_bg {
+        Some(bg) => Style::default().bg(user_message_bg(bg)),
+        None => Style::default(),
+    }
+}
+
+#[allow(clippy::disallowed_methods)]
+pub fn user_message_bg(terminal_bg: (u8, u8, u8)) -> Color {
+    let top = if is_light(terminal_bg) {
+        (0, 0, 0)
+    } else {
+        (255, 255, 255)
+    };
+    best_color(blend(top, terminal_bg, 0.1))
+}