Function into_subrun 

fn into_subrun(
    font: Font,
    text: String,
    glyphs: Vec<ShapedGlyph>,
    upem: u16,
    size_pt: f32,
) -> WordSubRun

Single-sub-run packaging when no fallback retry was needed. Skips the rebasing branch: the input glyphs already have cluster offsets relative to local_text (which is the full parent text).