fix(rss): emit podcast categories as text attributes

[cpruijsen]

Summary

• Fixes iTunes Category has invalid format #229: with podcast: true and category: "History", RSS 2.0 now emits <itunes:category text="History"/> (and the same shape for googleplay:category) instead of <itunes:category>History</itunes:category>.
• Provenance: reported by @UnprofessionalMartin on feed 5.1.0 against Apple's RSS guide. Still reproduced on current main. xml-js compact mode treats a raw string as element text; both podcast category tags were assigned that way in one block.
• Category text goes through existing sanitize() so names like Society & Culture become text="Society & Culture". xml-js does not escape & in attributes (same workaround as fix(atom): escape & in category attribute values #249 for Atom categories). Nested iTunes subcategories are not added; FeedOptions.category is still a single string.

Decision

• Chose: _attributes: { text } for both iTunes and Google Play, plus sanitize().
• Alternative: change only itunes:category; leave Google Play as element text; skip sanitizing because the reporter's example was History.
• Why: both tags were one assignment. Google Play's schema requires a text attribute on category. Apple's guide requires escaping ampersands, and several official category names contain &. xml-js will not escape those once the value is an attribute.
• Happy to drop the Google Play change or the sanitizer if you want this limited to the iTunes element shape.

xml-js emits a self-closing tag with no space before />. Apple's examples include a space. That is equivalent XML; this renderer already serializes empty elements that way.

Test plan

• pnpm test src/__tests__/rss2.spec.ts, new tests fail without the renderer change and pass with it
• pnpm test, 31 tests pass; existing podcast snapshot unchanged (it never set options.category)
• pnpm lint and pnpm format clean on src