[CT-15] Standardize event emission across all assetsup contract modules in opsce crate

[yusuftomilola]

Problem

Event emission in contracts/assetsup/src/ is inconsistent across modules. Some modules emit events; others silently mutate state. Backend services and indexers cannot reliably track on-chain state changes without consistent events.

Proposed Solution

Create contracts/opsce/src/events.rs that defines a standardized emit_event() helper and a complete set of typed event structs for all assetsup operations.

Acceptance Criteria

• Typed event structs defined for: AssetRegistered, AssetTransferred, AssetRetired, TokensMinted, TokensBurned, TokensTransferred, DividendsDistributed, VoteCast, LeaseCreated, LeaseReturned
• Each struct uses contracttype and has named fields (not raw bytes)
• emit_event(env, event) is a generic helper that calls env.events().publish()
• All modules in assetsup/src/ are updated to use the new event structs (or clearly document which modules still need updating)
• Unit tests verify that each event is emitted with the correct field values after the corresponding function call

[ummarig]

@ummarig has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Can I please be assigned this task

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ummarig to this issue.

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Congratulations, @ummarig! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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[drips-wave]

This issue has been completed by @ummarig as part of the Stellar Wave Program's 5th Wave 🥳

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