docs: document vault lifecycle event schema

[edochieblessing09-max]

Closes #354

Description

This PR resolves the issue to document the vault lifecycle event schema used in the axionvera-network smart contract. The documentation is necessary before SDK and dashboard indexing work can rely on contract events.

Changes Included

• Added docs/vault-events.md detailing the event topics, payload structures, and when each event is emitted.
• Included payload examples for init, deposit, withdraw, and claim events.
• Documented expected behavior for failed-call events.
• Added a section on SDK indexing expectations (filtering, type handling, event ordering, and failures).

No smart contract behavior was modified in this PR.

Acceptance Criteria Met

• Init event schema is documented.
• Deposit event schema is documented.
• Withdraw event schema is documented.
• Claim event schema is documented.
• Failed-call event behavior is documented.
• SDK indexing expectations are included.