Fix AuditGuardContract security-theater vulnerability

[kosisochukwu1234]

Closes #188

Fixes the security-theater vulnerability in AuditGuardContract by implementing genuine verification logic.

Changes made:

• AC-1: verify_context now reads the author's verification status from actual contract state (env.storage().instance().get(&author)) rather than trusting a caller-supplied boolean. A set_verified function was added to mutate this state.
• AC-2: validate_and_audit now uses env.crypto().ed25519_verify() to actually verify the provided signature against the payload. The function signature was updated to accept a public_key: BytesN<32> to enable proper cryptographic validation.
• AC-3: Added comprehensive unit tests in src/audit-guard/src/lib.rs proving that:
  • An invalid/forged signature is rejected (correctly triggers a panic, which is the expected rejection behavior in Soroban).
  • Empty payloads correctly return AuditGuardError::InvalidPayload.
  • Context verification correctly enforces state constraints instead of caller flags.

[N-thnI]

Merged — thanks for the contribution!

[N-thnI]

Also — if you've got a moment, a rating on GrantFox helps the project. Thanks again!