[ joinnextblock/attn-protocol ] refactor: ATTN extends City Protocol foundation
- ATTN relay now imports city-protocol/relay for base validation
- Moved Block validation to city-protocol (City owns kind 38808)
- ATTN go-core/validation now only contains ATTN-specific validation
- Updated ValidateATTNEvent to be the entry point for ATTN events
- Relay delegates City/supporting kinds to city-protocol/relay
ATTN Protocol is now an application layer on City Protocol.