Apple Secure Enclave DeviceCheck attestation

[jeremymanning]

Parent: #7

Implement Apple Secure Enclave attestation verification via Apple's DeviceCheck/App Attest API.

Files: src/verification/attestation.rs line 437
Requires: Apple Developer account for App Attest service

[jeremymanning]

Resolved by PR #58 (spec 003-stub-replacement). All 77 tasks complete, 489+ tests passing, CI green on Linux/macOS/Windows.