Fix ARM64 camera usage event decoding

[themartiano]

Summary

• model the V4L2 event payload union with its required 8-byte ARM64 alignment
• decode client-usage counts from the real payload instead of padding
• add regression coverage for every ABI offset and a live count payload

Verification

• python3 -m unittest guest.tests.test_native_camera_bridge
• make test
• live disposable VM: 60/60 1280x720 NV12 frames captured, 60 unique hashes, one expected black priming frame, FaceTime HD Camera activated
• producer restart while the consumer remained active recovered through SEND_INITIAL