[documentation] Expand core-loop.md with testnet contract deploy steps

[yinkscss]

Context

docs/core-loop.md describes local API registration and webhook simulation but stops short of real testnet contract deploy and RPC submission.

Problem

Contributors cannot validate the full loop on testnet without reading multiple files and scripts.

Proposed scope

• Extend core-loop.md with deploy-contract.sh walkthrough
• Document setting ESCROW_CONTRACT_ID and GATEWAY_SECRET_KEY
• Add expected on-chain explorer links and payout verification steps

Acceptance criteria

• Testnet section covers contract deploy and initialize
• Gateway env vars documented with example values
• End-to-end merge payout verifiable on testnet
• Cross-links attestation RPC implementation issue

References

• docs/core-loop.md
• scripts/deploy-contract.sh
• crates/gateway/src/attestation.rs
• .env.example