feat: implement podcast:image tag for Phase 8

[nathangathright]

Overview

This PR implements the new podcast:image tag as part of Phase 8 of the podcast namespace specification. This tag allows for delivering images of various sizes and use cases, expanding beyond the traditional square channel or episode art.

Changes Made

🏗️ Core Implementation

• Phase8PodcastImage type with all required attributes:

  • href (required): Image URL
  • alt (recommended): Accessibility text
  • aspect-ratio (recommended): CSS aspect ratio syntax (e.g., "1/1", "16/9", "4/1")
  • width (recommended): Width in pixels
  • height (optional): Height in pixels
  • type (optional): MIME type (supports video/mp4, image/jpeg, etc.)
  • purpose (optional): Space-separated tokens for use cases

• Parser implementation in phase-8.ts with proper validation

• Type definitions updated in types.ts for both FeedObject and Episode interfaces

• Phase registration for both feed and item processing

🧪 Testing

• Comprehensive test coverage (10 test cases) covering:
  • Basic usage with only href
  • All attributes together
  • Individual attribute extraction (alt, aspect-ratio, purpose)
  • Video content with type="video/mp4"
  • Accessibility features
• Example fixture demonstrating real-world usage

📋 Specification Compliance

• ✅ Multiple images per channel/item (as specified)
• ✅ Purpose tokens for indicating intended use cases
• ✅ Aspect ratio support with CSS syntax
• ✅ Video content support via type attribute
• ✅ Accessibility via alt attribute
• ✅ Platform targeting via custom purpose tokens

Usage Examples

Basic square artwork:

<podcast:image href="https://example.com/artwork.jpg" />

Social media optimized:

<podcast:image 
  href="https://example.com/banner.jpg" 
  alt="Podcast banner for social sharing"
  aspect-ratio="16/9" 
  width="1920" 
  height="1080" 
  purpose="artwork social"/>

Video content:

<podcast:image 
  href="https://example.com/animated.mp4" 
  type="video/mp4" 
  aspect-ratio="9/16" 
  purpose="artwork canvas"/>

Testing

All tests pass including the new Phase 8 tests:

• ✅ 10 new test cases for podcast:image
• ✅ All existing tests continue to pass
• ✅ No linting errors

Related

• Implements Phase 8 specification for podcast:image tag
• Replaces the deprecated podcast:images tag functionality
• Cross-compatible with existing itunes:image tags

Checklist

• TypeScript types defined
• Parser implementation complete
• Phase registration added
• Tests written and passing
• Example fixture created
• All attributes supported per spec
• Documentation updated
• No breaking changes