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/console.go

@@ -25,11 +25,11 @@ import (
 // It restores a Firecracker clone from the golden snapshot, connecting its serial
 // console (ttyS0) to a PTY, then serves the PTY master on a Unix socket at
 // {cloneDir}/console.sock for the lifetime of the VM.
-func RunConsoleProxy(cfg Config, id int, tapName string) error {
+func RunConsoleProxy(cfg Config, id int, tapName, tag string) error {
 	logger := log.WithField("component", fmt.Sprintf("console-proxy[%d]", id))
 
 	cloneDir := filepath.Join(cfg.BaseDir, "clones", strconv.Itoa(id))
-	goldenDir := filepath.Join(cfg.BaseDir, "golden")
+	goldenDir := filepath.Join(cfg.BaseDir, "golden", tag)
 	sockPath := filepath.Join(cloneDir, "api.sock")
 	consoleSockPath := filepath.Join(cloneDir, "console.sock")
 	sharedMem := filepath.Join(goldenDir, "mem")