Manage image artifacts and show VM create progress

Stop relying on ad hoc rootfs handling by adding image promotion, managed work-seed fingerprint metadata, and lazy self-healing for older managed images after the first create.

Rebuild guest images with baked SSH access, a guest NIC bootstrap, and default opencode services, and add the staged Void kernel/initramfs/modules workflow so void-exp uses a matching Void boot stack.

Replace the opaque blocking vm.create RPC with a begin/status flow that prints live stages in the CLI while still waiting for vsock health and opencode on guest port 4096.

Validate with GOCACHE=/tmp/banger-gocache go test ./... and live void-exp create/delete smoke runs.

internal/guestnet/guestnet.go

@@ -0,0 +1,30 @@
+package guestnet
+
+import _ "embed"
+
+const (
+	GuestScriptPath     = "/usr/local/libexec/banger-network-bootstrap"
+	SystemdServiceName  = "banger-network.service"
+	VoidCoreServicePath = "/etc/runit/core-services/20-banger-network.sh"
+)
+
+var (
+	//go:embed assets/bootstrap.sh
+	bootstrapScript string
+	//go:embed assets/systemd.service
+	systemdService string
+	//go:embed assets/void-core-service.sh
+	voidCoreService string
+)
+
+func BootstrapScript() string {
+	return bootstrapScript
+}
+
+func SystemdServiceUnit() string {
+	return systemdService
+}
+
+func VoidCoreService() string {
+	return voidCoreService
+}