feat: Add multi-arch support

- Support building for multiple platforms with --platform flag
- Accept comma-separated platforms or multiple --platform flags
- Default to building for linux/amd64 and linux/arm64
- Add ARM runner (ubuntu-24.04-arm) to CI matrix
- Create manifest list for multi-arch images (TODO: push to registry)
- Update integration tests to handle multi-platform builds
- Add cross-compilation setup instructions to README
- Add .cargo/config.toml for local development

Breaking changes:
- Default behavior now builds for multiple platforms (amd64+arm64)
- Use --platform linux/amd64 to build for single platform

src/lib.rs

@@ -2,6 +2,7 @@ pub mod builder;
 pub mod cli;
 pub mod config;
 pub mod image;
+pub mod manifest;
 pub mod registry;
 
 pub use anyhow::Result;