Function encode_css_pixel_as_i16 

pub fn encode_css_pixel_as_i16(prop: &CssPropertyValue<PixelValue>) -> i16

Resolve a CssPropertyValue to an i16 ×10 encoding. Only handles Exact(px(...)) values. Everything else → sentinel. For the compact cache builder, we only pre-resolve absolute pixel values. Relative units (em, %, etc.) get sentinel and fall back to the slow path.