- Added CharacterCardV3 struct to handle v3 format
- Created From<CharacterCardV3> for CharacterCardV2Data conversion
- Updated read_character_card_from_png() to detect and parse both v2 and v3 specs
- V3 cards have top-level fields + nested data object
- Maintains full backward compatibility with v2 cards
- Export still uses v2 format for maximum compatibility
