feat(markdown): emit asset:N hrefs for embedded images

[alexiskowalski]

Summary

Fixes #63: embedded assets already appear as Inline::Image { source: Asset(id) } in the document model, but Markdown dropped them to alt text only (or nothing when alt was empty), so to_markdown_bytes() lost positional image markers.

Changes

• Render ImageSource::Asset as ![alt](asset:N), where N is the index into Document.assets.
• Keep ImageSource::Unavailable as alt-text-only (no bytes to reference).
• Document the asset:N scheme in the README.
• Update Markdown snapshots that previously collapsed embedded images to bare alt text.

Callers can rewrite asset:N after writing asset files out, or keep using the document model for the bytes. This is complementary to approaches that inject resolved URLs.

Test plan

• Unit tests for asset href rendering (with and without alt).
• Snapshot updates for docx/odt/rtf/doc/epub fixtures that embed pictures.
• cargo test --locked render::markdown
• cargo test --locked --test snapshots
• CI