[CT-24] Implement transfer restriction whitelist management in assetsup in opsce crate

[yusuftomilola]

Problem

contracts/assetsup/src/transfer_restrictions.rs references a whitelist concept but has no functions to add, remove, or query addresses on the whitelist. Transfer restriction checks cannot function without the underlying whitelist management API.

Proposed Solution

Create contracts/opsce/src/whitelist.rs. Implement add_to_whitelist(), remove_from_whitelist(), is_whitelisted(), and a toggle to enable/disable whitelist enforcement per asset.

Acceptance Criteria

• add_to_whitelist(env, asset_id, address) adds an address (admin only)
• remove_from_whitelist(env, asset_id, address) removes an address (admin only)
• is_whitelisted(env, asset_id, address) returns a bool
• set_whitelist_enabled(env, asset_id, enabled) toggles enforcement (admin only)
• When enforcement is enabled, transfer_tokens() must call is_whitelisted() on both sender and recipient
• get_whitelist(env, asset_id) returns the full list of whitelisted addresses as a Vec
• Unit tests cover: add, remove, transfer allowed, transfer blocked, enforcement toggle

[nafsonig]

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

Let me implement this task as soon as possible

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

[drips-wave]

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

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