[Contracts] Make giveaway randomness cryptographically secure

[Kaylahray]

Description

The contract README explicitly notes that the current winner selection uses env.prng(), which is not cryptographically secure enough for production fairness.

User Story

As a giveaway creator,
I want to select winners using stronger randomness,
so that participants cannot predict or influence the result.

Requirements and Context

• Files: contracts/geev-core/src/giveaway.rs, contracts/geev-core/src/test.rs
• Replace ledger PRNG winner selection with a stronger randomness source or commit-reveal pattern
• Preserve the existing giveaway lifecycle and winner indexing logic
• Keep tests for deterministic verification

Suggested Implementation

// WINNER SELECTION
// 1. Replace env.prng().gen::<u64>() with a stronger entropy source.
// 2. Use the entropy to compute the winner index.
// 3. Preserve giveaway status transitions and event emission.

Acceptance Criteria

• Winner selection no longer depends on the current ledger PRNG alone.
• Existing giveaway lifecycle behavior remains intact.
• Tests continue to pass with deterministic coverage.

Submission Guidelines

• Branch: feat/secure-giveaway-randomness
• Depends on: existing giveaway lifecycle
• PR: feat(contracts): harden giveaway winner randomness

[morapay-app]

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

I’d like to work on this issue because improving randomness is critical for fairness in the giveaway system, and the current env.prng() approach is not secure enough for production use.

I can replace it with a stronger, verifiable randomness mechanism using a commit-reveal pattern or an oracle-based approach like Randamu. This will ensure winner selection cannot be predicted or manipulated, while preserving the existing giveaway lifecycle and keeping tests deterministic.

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

[Neziahtech]

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

hi maintainer, would love to work on this issue

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