Pull Request: Track Beneficiary and Delegate Changes (#27, #48

[JamesEjembi]

This PR implements the missing event emissions required for the backend indexer to track ownership and permission shifts within individual vesting vaults. By emitting these events, we ensure a "perfect audit trail" as the vault state evolves mid-stream.

🎯 Key Changes
Beneficiary Tracking: Added BeneficiaryUpdated event emission in the update_beneficiary function.

Delegate Tracking: Added DelegateUpdated event emission in the update_delegate function.

Schema Standardization: Events follow the standard (Symbol, Val) format for easy parsing by the Stellar indexer.

💻 Implementation Snippet (Soroban Rust)
The following logic was integrated into the vault administration functions:

Rust
// Inside the update_beneficiary function
let topics = (symbol_short!("vesting"), symbol_short!("ben_upd"), vault_id);
env.events().publish(topics, (old_beneficiary, new_beneficiary));

// Inside the update_delegate function
let topics = (symbol_short!("vesting"), symbol_short!("del_upd"), vault_id);
env.events().publish(topics, (old_delegate, new_delegate));
✅ Acceptance Criteria Checklist
[x] Event Emission: BeneficiaryUpdated(vault_id, old_address, new_address) is published on successful transfer.

[x] Delegate Tracking: DelegateUpdated(vault_id, old_delegate, new_delegate) is published when permissions change.

[x] Audit Integrity: Events include the vault_id as a topic for efficient filtering by the backend.

🧪 How to Verify
Run Tests:

Bash
cargo test
Verify Event Log:
Use the Soroban CLI to invoke the update functions and check the events output:

Bash
soroban events --last-n 2 --network standalone
🔗 Linked Issues
Closes #27
Closes #48