Function build_shaped_text 

pub fn build_shaped_text(
    shaper: &mut dyn TextShaper,
    display: &str,
    size_px: f32,
    opts: ShapeOptions,
    shape_version: u64,
) -> Option<ShapedText>

Pure, headless-testable shaping builder (O2): shape display once at size_px under opts, bake the caret metrics, and package the result.

An empty / unshapeable display yields a ShapedText with an empty run and empty geometry (rather than None), so an empty focused input still carries a stable ShapedText the caret / IME consumers can read and the producer can version-cache. The Option is retained for API symmetry; it is presently always Some.