Implement True Cross-Compilation Support

[spboyer]

Description

Complete the cross-compilation support to actually build for different architectures, not just set metadata.

Current State

Currently sets platform metadata but doesn't perform actual cross-compilation.

Proposed Implementation

• Integrate with cross-compilation tools
• Support QEMU for emulation
• Native ARM64 builds
• Platform-specific base image selection

Use Cases

• Building ARM64 images on x86_64 machines
• CI/CD multi-platform builds
• Apple Silicon support

Priority

High - Critical for modern cloud deployments.

Parent: #47