Implement invoice merkle proof system

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Verifying a specific payment exists among thousands requires scanning the entire payments list. This issue adds build_payment_merkle() that computes a Merkle root over all payment entries and stores it on-chain, with verify_payment_merkle() allowing efficient single-payment verification using a proof path.

Technical Context

Involves lib.rs. Add build_payment_merkle(env, invoice_id) that hashes each (payer, amount) pair using env.crypto().sha256(), builds a binary Merkle tree, and stores the root in persistent key ("merkle", invoice_id). Add get_merkle_root(env, invoice_id) -> BytesN<32>.

Acceptance Criteria

• build_payment_merkle() computes and stores Merkle root
• get_merkle_root() returns stored root
• Same payment set always produces same root (deterministic)
• Root changes after new payment added and merkle rebuilt
• Test builds merkle for 4 payments and verifies root is non-zero and consistent
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[Essence-3]

@Essence-3 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi, I have a solution for this issue and I would love to work on it.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Essence-3 to this issue.

[Opulencechuks]

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

I would like to work on this issue because i can implement it having worked on similar issues

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