feat: implement contract event mapper for role events

[edochieblessing09-max]

Closes #148

Description

This PR implements the contract event mapper for Aegis protocol role events, fulfilling the requirement for Dashboard and SDK consumers to parse these events safely.

Changes Included

• Event Types: Added RoleContractEvent to the AegisContractEvent types and added 'role' to ContractEventKind.
• Topics & Aliases: Registered role_grant and role_revoke topics along with aliases (grant_role, role_granted, revoke_role, role_revoked) in topics.ts.
• Decoder Utility: Added decodeRoleEvent implementation in decoder.ts to map role_grant and role_revoke topics into the typed RoleContractEvent object safely while handling missing fields.
• Fixtures: Added roleGrant and roleRevoke deterministic test fixtures.
• Tests: Added tests covering known role events to events-decoder.test.ts.
• Documentation: Updated contract-events.md to reflect role event support and compatibility.

Acceptance Criteria Met

• Contract event mapper updated.
• Compliance, minting, transfer, role, and asset events are supported.
• Unknown events are handled safely.
• Fixtures are added.
• Tests cover known and unknown events.
• Docs explain event compatibility.