feat: implement initial sell-key contract flow

[edehvictor]

Closes #3

Changes

• add an initial sell_key contract entrypoint for creator keys
• decrement seller key balance and creator supply on successful sell
• decrement holder count when a seller exits their final key position
• return structured errors for invalid sell states, including unregistered creators and insufficient balances
• add integration tests for successful and invalid sell flows

Testing

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

Notes

• this PR focuses on the first sell-key state transition only
• payment routing and sell event expansion can be handled in follow-up issues