Backgrounds stay inside their box

A rendering fix that shows up on almost every code block and every nav bar. When a box holds text that does NOT wrap - preformatted text, a `white-space:nowrap` element, or one very long word - the text spills past the box's right edge, exactly as it does in every browser. But Floati was dragging the box's BACKGROUND (and its border) out with the text, painting a coloured stripe far wider than the box. Now the background and border stay at the box's real width and the text spills over them, matching the reference browser. The cause was subtle: a line of text is painted as little per-word pieces that share the element's identity, and each piece was re-painting the whole element's background around itself - harmless while the text stayed inside the box, wrong the moment it overflowed. The pieces now paint only their text; the box paints its own background once, at the right width. Measured against the reference browser across the whole rendering test suite the change moves only toward the reference browser, and the engine gate holds at 6375 of 6375.