docs: add round 175's background-repeat paint-cost finding

[tannevaled]

Summary

• Extends the Security section with engine PR fix(paint): normalise background-repeat tiles once, not once per pixel engine#288 (v0.7.2): a real page with repeating background-image sections could take over a minute to paint because of an avoidable per-pixel allocation/conversion cost, not a crafted attack.

Test plan

• Prose-only Markdown change, reviewed by eye.