Build invoice payment proof batch verification

[Kingsman-99]

Label: complexity: high
Points: 200

Description

verify_payment_proof() (separate issue) verifies one proof at a time. For off-chain systems reconciling many payments at once, batching reduces the number of contract calls needed.

Technical Context

Involves lib.rs — verify_payment_proofs_batch(env, proofs: Vec<PaymentProof>) -> Vec<bool>, calling the single-proof verification logic in a loop and returning results in the same order, capped at 20 proofs per call.

Acceptance Criteria

• Returns a Vec<bool> the same length and order as the input proofs
• Panics with "batch limit exceeded" for more than 20 proofs
• A single invalid/stale proof in the batch doesn't prevent verification of the others
• Test with a mixed batch of valid and stale proofs verifies correct per-entry results
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[grantfox-oss]

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@johnsaviour56-ship-it's PR #256 was approved and merged by @Kingsman-99.

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