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/test_backend.rs

@@ -0,0 +1,124 @@
+use std::fmt::{self};
+use std::io::Write;
+use std::io::{self};
+
+use ratatui::prelude::CrosstermBackend;
+
+use ratatui::backend::Backend;
+use ratatui::backend::ClearType;
+use ratatui::backend::WindowSize;
+use ratatui::buffer::Cell;
+use ratatui::layout::Position;
+use ratatui::layout::Size;
+
+/// This wraps a CrosstermBackend and a vt100::Parser to mock
+/// a "real" terminal.
+///
+/// Importantly, this wrapper avoids calling any crossterm methods
+/// which write to stdout regardless of the writer. This includes:
+/// - getting the terminal size
+/// - getting the cursor position
+pub struct VT100Backend {
+    crossterm_backend: CrosstermBackend<vt100::Parser>,
+}
+
+impl VT100Backend {
+    /// Creates a new `TestBackend` with the specified width and height.
+    pub fn new(width: u16, height: u16) -> Self {
+        Self {
+            crossterm_backend: CrosstermBackend::new(vt100::Parser::new(height, width, 0)),
+        }
+    }
+
+    pub fn vt100(&self) -> &vt100::Parser {
+        self.crossterm_backend.writer()
+    }
+}
+
+impl Write for VT100Backend {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.crossterm_backend.writer_mut().write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.crossterm_backend.writer_mut().flush()
+    }
+}
+
+impl fmt::Display for VT100Backend {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.crossterm_backend.writer().screen().contents())
+    }
+}
+
+impl Backend for VT100Backend {
+    fn draw<'a, I>(&mut self, content: I) -> io::Result<()>
+    where
+        I: Iterator<Item = (u16, u16, &'a Cell)>,
+    {
+        self.crossterm_backend.draw(content)?;
+        Ok(())
+    }
+
+    fn hide_cursor(&mut self) -> io::Result<()> {
+        self.crossterm_backend.hide_cursor()?;
+        Ok(())
+    }
+
+    fn show_cursor(&mut self) -> io::Result<()> {
+        self.crossterm_backend.show_cursor()?;
+        Ok(())
+    }
+
+    fn get_cursor_position(&mut self) -> io::Result<Position> {
+        Ok(self.vt100().screen().cursor_position().into())
+    }
+
+    fn set_cursor_position<P: Into<Position>>(&mut self, position: P) -> io::Result<()> {
+        self.crossterm_backend.set_cursor_position(position)
+    }
+
+    fn clear(&mut self) -> io::Result<()> {
+        self.crossterm_backend.clear()
+    }
+
+    fn clear_region(&mut self, clear_type: ClearType) -> io::Result<()> {
+        self.crossterm_backend.clear_region(clear_type)
+    }
+
+    fn append_lines(&mut self, line_count: u16) -> io::Result<()> {
+        self.crossterm_backend.append_lines(line_count)
+    }
+
+    fn size(&self) -> io::Result<Size> {
+        let (rows, cols) = self.vt100().screen().size();
+        Ok(Size::new(cols, rows))
+    }
+
+    fn window_size(&mut self) -> io::Result<WindowSize> {
+        Ok(WindowSize {
+            columns_rows: self.vt100().screen().size().into(),
+            // Arbitrary size, we don't rely on this in testing.
+            pixels: Size {
+                width: 640,
+                height: 480,
+            },
+        })
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.crossterm_backend.writer_mut().flush()
+    }
+
+    fn scroll_region_up(&mut self, region: std::ops::Range<u16>, scroll_by: u16) -> io::Result<()> {
+        self.crossterm_backend.scroll_region_up(region, scroll_by)
+    }
+
+    fn scroll_region_down(
+        &mut self,
+        region: std::ops::Range<u16>,
+        scroll_by: u16,
+    ) -> io::Result<()> {
+        self.crossterm_backend.scroll_region_down(region, scroll_by)
+    }
+}