Gemini medium-priority review on #5629 suggested folding the two
`[data-streamdown="code-block"]` rules into one selector block so the
applied styles are visible in one place. Move `max-width: 100%` +
`overflow-x: hidden` into the existing
`content-visibility / contain-intrinsic-size` rule alongside the
explanatory comment, keeping the inner `<pre>` overflow-x rule next to
it for readability. No specificity / !important changes.
74b1fa7513