Dual-Layer Isolation.

/// Pluggable VMM backend trait.
/// Everything above the backend -- OCI pipeline, server,
/// result protocol -- is backend-agnostic.
#[async_trait]
pub trait VmmBackend: Send + Sync {
    /// Run a VM and return the result.
    async fn run(
        &self, config: &VmConfig, worker_bin: &Path,
    ) -> Result<VmResult, HotcellError>;

    /// Run a VM with streaming console output.
    async fn run_streaming(
        &self, config: &VmConfig, worker_bin: &Path,
        tx: broadcast::Sender<StreamEvent>,
    ) -> Result<VmResult, HotcellError>;

    /// Create a persistent VM that outlives a single request.
    async fn create_persistent(
        &self, config: &VmConfig, worker_bin: &Path,
    ) -> Result<Box<dyn PersistentVmHandle>, HotcellError>;

    /// Human-readable backend name.
    fn name(&self) -> &'static str;
}

// Four implementations in separate crates:
// - hotcell_libkrun::LibkrunBackend      (macOS + Linux)
// - hotcell_firecracker::FirecrackerBackend  (Linux only)
// - hotcell_ch::ChBackend                (Linux only)
// - hotcell_qemu::QemuBackend            (Linux only)