feat(creator-keys): emit richer registration events

[edehvictor]

Closes #2

Changes

• expand the creator registration event into a stable typed payload for downstream indexing
• include the creator address as an indexed event topic
• expose creator, handle, supply, and holder_count in the registration event data
• document the registration event structure in code comments
• update event tests to verify the richer payload and indexed creator topic

Testing

• cargo fmt --all -- --check
• CI will run:
  • cargo clippy --workspace --all-targets -- -D warnings
  • cargo test --workspace

Notes

• this PR focuses only on improving registration event structure for indexers
• buy and sell event enrichment can continue in follow-up issues