[CT-23] Implement dividend streaming in assetsup/dividends.rs in opsce crate

[yusuftomilola]

Problem

contracts/assetsup/src/dividends.rs implements lump-sum dividend distribution but does not support continuous streaming. Token holders must wait for periodic distributions rather than earning dividends proportionally per second.

Proposed Solution

Create contracts/opsce/src/dividend_stream.rs. Implement a streaming dividend mechanism where a pool of funds is distributed linearly over a defined period, and holders can claim their accrued share at any time.

Acceptance Criteria

• start_dividend_stream(env, asset_id, total_amount, duration_seconds) initiates a stream (admin only)
• calculate_claimable(env, asset_id, holder) returns tokens earned since last claim based on proportional share and elapsed time
• claim_stream_dividends(env, asset_id) transfers accrued amount to caller and updates last-claim timestamp
• end_dividend_stream(env, asset_id) stops the stream and returns unclaimed funds to admin
• Emits stream_started, dividends_claimed, and stream_ended events
• Unit tests cover: proportional share calculation, multiple claims, zero-balance holder, stream end

[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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