Deduplicate image description by moving it to img alt text

[Br1an67]

Summary

Fix image descriptions appearing twice in output when use_llm=True — once as a separate text paragraph and once embedded with the image.

When the LLM generates an image description, Picture/Figure.assemble_html() creates a <p role='img'> paragraph with the description. When images are also being extracted, this description paragraph ends up in the output alongside the <img> tag, causing duplication.

The fix extracts the description from the <p role='img'> tag, moves it into the <img> tag's alt attribute, and removes the separate paragraph. This produces clean output like:

Instead of the duplicated:

Image ... description: Diagram showing battery modules connected in parallel.

Closes #955

Changes

• Modified HTMLRenderer.extract_html() to extract description from p[role='img'] and use it as the <img> alt text
• Description paragraph is removed from content to prevent duplication
• Alt text is properly HTML-escaped