Add a contract event reference for indexers

[ChaoLing140]

Backlog ID: #324

• Context: Indexers need a stable source of truth for emitted events and their meaning.
• Acceptance criteria:
  • All externally relevant events are listed with field descriptions.
  • Event ordering and lifecycle assumptions are documented.
  • The document links to the contract modules that emit each event.
• Suggested tests:
  • Cross-check against contract event definitions.
  • Doc review for field completeness.