feat(parser): support phase 8 podcast follow

[nathangathright]

Summary

• Add parser support for the phase 8 podcast:follow tag.
• Expose the parsed follow endpoint as feed.podcastFollow.url.
• Report phase 8 support for follow only when a valid url attribute is present.
• Add tests for missing tags, missing/empty URLs, multiple tags, and known live-feed examples from Podnews and PRX/Radiotopia.

Verification

• npm test -- src/parser/phase/__test__/phase-8.test.ts
• Live feed check: https://podnews.net/rss parsed https://podnews.net/podcast/i8xe9/follow.json
• Live feed check: http://feeds.radiotopia.fm/radiotopia-showcase parsed https://f.prxu.org/72/subscribelinks.json

[nathangathright]

Follow-up validation after the latest force-push:

• Tightened podcast:follow handling to select the first valid HTTP(S) URL and ignore malformed/unsupported url values.
• Added regression coverage for malformed URLs and invalid-first/valid-second duplicate tags.
• Re-ran npm test -- --runTestsByPath src/parser/phase/__test__/phase-8.test.ts --runInBand, npm run lint:typecheck, npm run lint:lint, and npm run build.
• Re-tested the 24 live podcast:follow feeds discovered in the adoption crawl: 23 parsed successfully. The remaining feed still contains a raw follow tag but is ~31 MB and exceeds the parser's existing 20 MiB max feed size before tag extraction.

[nathangathright]

Additional follow-up after code-review recheck:

• The re-review was clean except for a minor test gap around explicit http:// acceptance.
• Added a positive HTTP follow URL test.
• Re-ran npm test -- --runTestsByPath src/parser/phase/__test__/phase-8.test.ts --runInBand, npm run lint:typecheck, npm run lint:lint, and npm run build successfully.