[FEATURE] Add KYC attestation contract: users can bypass KYC by calling contracts directly

[derbydaniels9-cpu]

Summary

There is no on-chain user identity or KYC attestation contract. KYC status is stored only in the backend database, meaning it can be bypassed by directly calling contract functions with a Stellar keypair that was never KYC-verified.

Impact

If a user obtains a Stellar keypair and the contract address, they can interact directly with the savings or swap contracts without going through the app's KYC flow. This violates AML/KYC compliance requirements.

Proposed contract

A Soroban KYC attestation contract that:

1. Stores a mapping of Address -> KycLevel (NONE, BASIC, FULL).
2. Can only be written to by a trusted attestor address (the backend's signing key).
3. Is read by other contracts (savings lock, swap) to gate access:

pub trait KycRegistry {
    fn attest(env: Env, attestor: Address, user: Address, level: u32);
    fn revoke(env: Env, attestor: Address, user: Address);
    fn get_kyc_level(env: Env, user: Address) -> u32;
    fn require_kyc(env: Env, user: Address, min_level: u32); // panics if not met
}

Other contracts call kyc_contract.require_kyc(&env, &user, KYC_BASIC) before allowing deposits above a threshold.

Files affected:

• New: src/kyc_registry/lib.rs
• New: src/kyc_registry/Cargo.toml