Add async verification utilities (v1.x)

[AI-God-Dev]

Problem

Verification cost is non-trivial at high throughput.

Scope

• Provide helper to queue and batch verification tasks.
• Add concurrency controls and timeouts.

Acceptance Criteria

• Utility module documented with examples.
• Works with existing AttestationVerifier behavior.

Notes

Focus on SDK-only support; no network calls.