feat: multi-distro support and tagged golden snapshots

Add Alpine, Debian, and Ubuntu rootfs support to `init [distro]`.
Golden snapshots are now namespaced under `golden/<tag>/` so multiple
baselines can coexist. `spawn [tag] [N]` selects which snapshot to
clone from. Systemd-based distros (Debian, Ubuntu) get a fc-net-init
systemd unit; Alpine keeps its inittab-based init.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

orchestrator/config.go

@@ -2,6 +2,7 @@ package orchestrator
 
 import (
 	"os"
+	"path/filepath"
 	"strconv"
 )
 
@@ -11,7 +12,7 @@ type Config struct {
 	BaseDir    string // working directory for all state
 	Kernel     string // path to vmlinux
 	KernelURL  string // URL to download vmlinux if Kernel file is missing
-	Rootfs     string // path to base rootfs.ext4
+	CustomRootfs string // Custom path to rootfs if FC_ROOTFS is set
 	VCPUs      int64
 	MemMiB     int64
 	Bridge       string // host bridge name, or "none" to skip networking
@@ -38,10 +39,18 @@ func DefaultConfig() Config {
 	c.Kernel = envOr("FC_KERNEL", c.BaseDir+"/vmlinux")
 	c.KernelURL = envOr("FC_KERNEL_URL",
 		"https://s3.amazonaws.com/spec.ccfc.min/firecracker-ci/20260408-ce2a467895c1-0/x86_64/vmlinux-6.1.166")
-	c.Rootfs = envOr("FC_ROOTFS", c.BaseDir+"/rootfs.ext4")
+	c.CustomRootfs = os.Getenv("FC_ROOTFS")
 	return c
 }
 
+// RootfsPath returns the path to the root filesystem depending on the requested distribution.
+func (c Config) RootfsPath(distro string) string {
+	if c.CustomRootfs != "" {
+		return c.CustomRootfs
+	}
+	return filepath.Join(c.BaseDir, "rootfs-"+distro+".ext4")
+}
+
 func envOr(key, fallback string) string {
 	if v := os.Getenv(key); v != "" {
 		return v