fix(payment): enforce single-node proof verification

[mickvandijke]

Summary

• Enforce the full single-node payment proof validation path before accepting new stores.
• Validate the quote set against the content address, quoted peer identities, ML-DSA signatures, and this node self-inclusive local close-group view.
• Replace the generic SingleNodePayment::verify() call with verifier-owned completedPayments(quote.hash()) checks for amount and recipient binding.

SemVer: patch

Requested Checklist Coverage

• Proof contains exactly CLOSE_GROUP_SIZE quote entries.
• Quote peer IDs are distinct.
• Every quote is for the uploaded content address.
• Every quote is signed by the peer that claims it.
• Every quoted peer is present in this node valid local close-group view for the content.
• The paid quote is selected from the already validated quote set, including median-price tie handling.
• completedPayments(quote.hash()) shows an amount at least 3 * quote.price.
• completedPayments(quote.hash()) recipient prefix matches the quote rewards address.

Recipient Binding Note

PaymentVaultV2.completedPayments currently stores the recipient as bytes16, not the full 20-byte EVM address. This PR verifies the stored 16-byte prefix against the quote rewards address. Full 20-byte equality would require a contract/ABI change or event-based reconstruction.

Tests

• cargo fmt --all -- --check
• cargo test payment::verifier
• cargo clippy --all-targets --all-features -- -D warnings

Commit

• 23d15b5 fix(payment): enforce single-node proof verification

[Copilot]

Pull request overview

Tightens single-node payment proof verification in PaymentVerifier by ensuring the proof’s quoted peers match the verifier’s local close-group view and by strengthening the on-chain validation (median-priced selection, recipient-prefix binding, and under/overpayment handling) against PaymentVault.completedPayments.

Changes:

• Adds local close-group membership enforcement for all quoted peers in single-node proofs.
• Replaces the previous SingleNodePayment::verify(...) path with explicit on-chain completedPayments(quoteHash) checks that handle median-price ties and validate rewards-address prefix + paid amount.
• Extends unit tests to cover close-group set validation and completed-payment amount/recipient-prefix checks.

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[Copilot]

Pull request overview

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Pull request overview

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Comments suppressed due to low confidence (1)

src/payment/verifier.rs:501

• The CPU-heavy ML-DSA signature verification runs before the single-node close-group check (which also fails fast if no P2PNode is attached). To reduce DoS surface and avoid wasting CPU when the proof will be rejected due to close-group / attachment constraints, consider running validate_single_node_close_group(...) (or at least the p2p_node attachment + local peer/rewards checks) before spawning the blocking signature verification task.

        // Verify quote signatures (CPU-bound, run off async runtime)
        let peer_quotes = payment.peer_quotes.clone();
        tokio::task::spawn_blocking(move || {
            for (encoded_peer_id, quote) in &peer_quotes {
                if !verify_quote_signature(quote) {