Authors hook: tests, an RSS icon, and small RSS polish

[RichardLitt]

Follow-up from #144. A pre-merge review surfaced a handful of smaller items that didn't need to block merging the feed itself but are worth a second pass once #144 is in.

• Add automated tests for the authors hook (.config/hooks/authors.py). The hook now has several helpers — _validate_authors, _first_paragraph, _author_html, _feed_description, and the post-build XML rewriter — and the review surfaced specific edge cases worth pinning down so they don't regress (e.g. names with apostrophes, ORCID validation, patterns whose first paragraph is a blockquote or code block).
• Use the RSS icon next to the "Subscribe" link rather than plain text. The Material theme already imports material/rss-box (it's the icon used for the "Awareness" tag), so we can reuse it for free.
• Standardise the subscribe wording across the three surfaces (home README, authors index, per-author pages). They currently use three slightly different phrasings; one consistent sentence would read better.
• Move the per-author subscribe line above the patterns list rather than below it. On someone like Ciara's page, the line currently sits below a long list of patterns and is easy to miss; putting it just under the affiliation/ORCID block makes it visible without scrolling.

None of these are urgent — the feed works correctly as of #144 — but they're the kind of small polish that adds up.