Core: Implement actual construction logic for Merkle Tree

[EDOHWARES]

Fix the TODO in core/src/merkle_tree.rs line 30 to properly hash nodes.

[Nuel-ship-it]

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

I will love to work on this issue.

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[drips-wave]

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

🧑‍💻 @Nuel-ship-it: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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