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/shimmer.rs

@@ -0,0 +1,80 @@
+use std::sync::OnceLock;
+use std::time::Duration;
+use std::time::Instant;
+
+use ratatui::style::Color;
+use ratatui::style::Modifier;
+use ratatui::style::Style;
+use ratatui::text::Span;
+
+use crate::color::blend;
+use crate::terminal_palette::default_bg;
+use crate::terminal_palette::default_fg;
+
+static PROCESS_START: OnceLock<Instant> = OnceLock::new();
+
+fn elapsed_since_start() -> Duration {
+    let start = PROCESS_START.get_or_init(Instant::now);
+    start.elapsed()
+}
+
+pub(crate) fn shimmer_spans(text: &str) -> Vec<Span<'static>> {
+    let chars: Vec<char> = text.chars().collect();
+    if chars.is_empty() {
+        return Vec::new();
+    }
+    // Use time-based sweep synchronized to process start.
+    let padding = 10usize;
+    let period = chars.len() + padding * 2;
+    let sweep_seconds = 2.0f32;
+    let pos_f =
+        (elapsed_since_start().as_secs_f32() % sweep_seconds) / sweep_seconds * (period as f32);
+    let pos = pos_f as usize;
+    let has_true_color = supports_color::on_cached(supports_color::Stream::Stdout)
+        .map(|level| level.has_16m)
+        .unwrap_or(false);
+    let band_half_width = 5.0;
+
+    let mut spans: Vec<Span<'static>> = Vec::with_capacity(chars.len());
+    let base_color = default_fg().unwrap_or((128, 128, 128));
+    let highlight_color = default_bg().unwrap_or((255, 255, 255));
+    for (i, ch) in chars.iter().enumerate() {
+        let i_pos = i as isize + padding as isize;
+        let pos = pos as isize;
+        let dist = (i_pos - pos).abs() as f32;
+
+        let t = if dist <= band_half_width {
+            let x = std::f32::consts::PI * (dist / band_half_width);
+            0.5 * (1.0 + x.cos())
+        } else {
+            0.0
+        };
+        let style = if has_true_color {
+            let highlight = t.clamp(0.0, 1.0);
+            let (r, g, b) = blend(highlight_color, base_color, highlight * 0.9);
+            // Allow custom RGB colors, as the implementation is thoughtfully
+            // adjusting the level of the default foreground color.
+            #[allow(clippy::disallowed_methods)]
+            {
+                Style::default()
+                    .fg(Color::Rgb(r, g, b))
+                    .add_modifier(Modifier::BOLD)
+            }
+        } else {
+            color_for_level(t)
+        };
+        spans.push(Span::styled(ch.to_string(), style));
+    }
+    spans
+}
+
+fn color_for_level(intensity: f32) -> Style {
+    // Tune fallback styling so the shimmer band reads even without RGB support.
+    if intensity < 0.2 {
+        Style::default().add_modifier(Modifier::DIM)
+    } else if intensity < 0.6 {
+        Style::default()
+    } else {
+        Style::default().add_modifier(Modifier::BOLD)
+    }
+}