• Added contracts/cross_contract/src/lib.rs:1 with a new Soroban cross-contract messaging contract: message queue, route registry, optional
  callbacks, rate limiting, audit trail, and atomic/non-atomic delivery modes.
• Added contracts/cross_contract/src/test.rs:1 with coverage for routing, callback execution, callback override, queue behavior, rate limiting,
  error propagation, and atomic rollback expectations.
• Registered the crate in Cargo.toml:3 and added contracts/cross_contract/Cargo.toml:1.

closes #260