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>

test_mmds.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+sock="/tmp/fctest.sock"
+rm -f "$sock"
+firecracker --api-sock "$sock" &
+FCPID=$!
+sleep 1
+
+# Configure MMDS backend
+curl --unix-socket "$sock" -i -X PUT "http://localhost/mmds/config" \
+  -H "Accept: application/json" -H "Content-Type: application/json" \
+  -d '{"version": "V1", "network_interfaces": ["1"], "ipv4_address": "169.254.169.254"}'
+
+# Put data
+curl --unix-socket "$sock" -i -X PUT "http://localhost/mmds" \
+  -H "Accept: application/json" -H "Content-Type: application/json" \
+  -d '{"ip": "10.0.0.2", "gw": "10.0.0.1", "dns": "1.1.1.1"}'
+
+# Read data
+curl --unix-socket "$sock" -i -X GET "http://localhost/mmds"
+
+kill $FCPID