docs: describe the DoS-hardening work from engine rounds 166-173

[tannevaled]

Summary

• The Security section covered only the raster-image megapixel cap and the dial-time SSRF guard — nothing about the DOM-depth stack-overflow caps or the cloneNode/innerHTML memory-exhaustion caps shipped across engine rounds 166-173.
• Adds a short breakdown (decoding / tree-depth recursion / memory exhaustion) and links to the engine's FIDELITY.md for the full measured writeups.

Test plan

• Rendered via mkdocs serve locally not required for a prose-only Markdown change; reviewed the rendered Markdown by eye.