[Contracts] Add manual or merit-based winner selection

[Kaylahray]

Description

The contracts README lists merit-based and manual selection as future giveaway modes, but the current implementation only supports a random winner path.

User Story

As a creator,
I want to choose winners manually or by merit,
so that I can run campaigns that are not purely random.

Requirements and Context

• Files: contracts/geev-core/src/types.rs, contracts/geev-core/src/giveaway.rs, contracts/geev-core/src/test.rs
• Add a selection mode for manual and merit-based winner resolution
• Validate that only the creator or authorized admin can finalize the result
• Keep the existing random flow available for default giveaways

Suggested Implementation

// SELECTION MODES
// 1. Add a selection_method field to giveaway state.
// 2. Support manual winner assignment and merit-based sorting.
// 3. Gate each mode with appropriate authorization.
// 4. Emit the same lifecycle events for downstream indexing.

Acceptance Criteria

• A giveaway can be finalized manually when configured.
• Merit-based selection can be enabled without breaking random selection.
• Unauthorized callers cannot change the winner.

Submission Guidelines

• Branch: feat/manual-merit-selection
• Depends on: giveaway state model
• PR: feat(contracts): add manual and merit-based winner selection

[Favourof]

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

Hi maintainer, I would like to work on this issue.

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

[SweetBoy-eth]

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

Hello maintainer, I can handle this issue.

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

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. Thank you!