- Add Firecracker backend for Linux with KVM support
- Add pluggable backend architecture with traits and factory
- Auto-download Wasmer and Firecracker binaries at runtime
- Auto-download kernel and rootfs assets for Firecracker
- Update install.sh to install Wasmer and Firecracker
- Add guest-agent for Firecracker VM communication via vsock
- Change default backend to Auto (prefers Firecracker on Linux, falls back to Wasmer)
- Add docs site with installation instructions