RSS enhancements

1. Deep Feature Categorization: Expand rssVisitor to categorize highlights into Baseline Newly Available, Widely Available, and Regressed to Limited Availability.
2. Browser Implementation Updates: Expand rssVisitor to extract and group browser version changes (e.g. Chrome 125, Safari 17.4).
3. Rich HTML Template & Permalinks: Update rssItemTemplate in rss_renderer.go to render feature permalinks (/features/{id}), MDN links, and styled baseline status badges.
4. Shared Summary Presentation Core: Extract a shared summary presentation package (lib/summaryrender) used by Email, Webhooks, and RSS to eliminate duplicated category and browser name formatting logic.

See the end of [this](https://github.com/GoogleChrome/webstatus.dev/pull/2594#pullrequestreview-4664562718) comment.

5. https://github.com/GoogleChrome/webstatus.dev/pull/2594#discussion_r3554495165

Start with rssVisitor and rss_renderer.go, including rssItemTemplate, then review the linked pull request discussion for the expected summary behavior. Trace how Email and Webhooks present summaries before considering the shared lib/summaryrender package. Done means RSS includes categorized and browser-version changes, feature and MDN links, styled baseline badges, and shared presentation logic is used across the listed outputs.

Written by the indexing model from the issue text.