/// A type that knows the pixel dimensions of its terminal cells.
///
/// # Physical vs. CSS pixels
///
/// On HiDPI / Retina displays the device pixel ratio (DPR) causes one
/// CSS pixel to map to multiple physical (device) pixels.
///
/// - **Physical pixels** (`cell_size_px`): the actual device pixels
/// occupied by a cell. Useful for pixel-perfect rendering and GPU work.
/// - **CSS pixels** (`cell_size_css_px`): the logical size as seen by
/// the browser layout engine. Useful for DOM positioning, mouse
/// coordinate translation, and canvas drawing.