Implement cross-chain bridge payment relay for invoices

[Kingsman-99]

Overview

Add support for accepting invoice payments relayed from other chains (e.g. Ethereum, Solana) via a trusted bridge contract. The StellarSplit contract must verify a signed relay proof before crediting the payment, so no funds can be injected without a valid cross-chain attestation.

Requirements

• Define a bridge_pay entry point that accepts a relay proof struct (BridgeProof) containing: source chain ID, source tx hash, sender address bytes, amount, and a bridge authority signature
• Verify the signature against a stored bridge_authority admin key before crediting
• Emit a BridgePaymentReceived event with full proof metadata
• Integrate with the existing payment flow — trigger auto-release if the invoice becomes fully funded
• Reject replayed proofs (store consumed tx hashes)
• Write unit tests covering: valid relay, invalid signature, replayed proof, wrong invoice, wrong amount

Acceptance Criteria

• bridge_pay entry point implemented and callable
• Signature verification passes for valid proofs and rejects tampered ones
• Replay protection: second call with same tx hash panics with AlreadyConsumed
• Auto-release triggers correctly when bridge payment fully funds an invoice
• All existing tests continue to pass
• New tests cover all acceptance criteria above
• cargo test --workspace passes with zero failures
• cargo clippy --all-targets -- -D warnings produces zero warnings

Definition of Done

All CI checks must pass before the PR is reviewed. PRs with failing tests or clippy warnings will not be merged.

[nurudeenheroic]

Hi! I'd love to work on this. I'm confident I can deliver a clean solution.

[Achievers-sketch]

Experienced building Soroban smart contracts, secure cross-chain payment relays, signature verification, replay protection, event handling, and comprehensive Rust test suites.