feat: add guest network autoconfiguration via Firecracker MMDS

Introduces optional per-clone IP assignment using the Firecracker Microvm
Metadata Service (MMDS). A background daemon (fc-net-init) is baked into
the rootfs during init and captured in the golden snapshot — on clone
resume it polls 169.254.169.254 and applies the IP/GW/DNS config injected
by the orchestrator immediately after snapshot restore.

- config.go: add AutoNetConfig bool (FC_AUTO_NET_CONFIG=1)
- orchestrator.go: embed fc-net-init daemon + MMDS link-local route in
  init script; set AllowMMDS: true on golden NIC; spawnOne/SpawnSingle
  accept net bool and propagate it via FC_AUTO_NET_CONFIG in proxy env
- console.go: set AllowMMDS: true on clone NIC; call configureMmds()
  after m.Start() when AutoNetConfig is enabled
- network.go: add configureMmds() — PUT /mmds with ip/gw/dns over the
  clone's Firecracker Unix socket
- serve.go: POST /clones accepts optional {"net": bool} body to override
  the global AutoNetConfig default per-request
- web/terminal.html: spawn button always sends {"net": true}
- docs/commands.md: document manual config + MMDS autoconfiguration

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

orchestrator/config.go

@@ -14,11 +14,12 @@ type Config struct {
 	Rootfs     string // path to base rootfs.ext4
 	VCPUs      int64
 	MemMiB     int64
-	Bridge     string // host bridge name, or "none" to skip networking
-	BridgeCIDR string // e.g. "172.30.0.1/24"
-	GuestPrefix string // e.g. "172.30.0" — clones get .10, .11, ...
-	GuestGW    string
-	BootArgs   string
+	Bridge       string // host bridge name, or "none" to skip networking
+	BridgeCIDR   string // e.g. "172.30.0.1/24"
+	GuestPrefix  string // e.g. "172.30.0" — clones get .11, .12, ...
+	GuestGW      string // default gateway for guest VMs
+	AutoNetConfig bool   // inject guest IP/GW/DNS via MMDS on clone start
+	BootArgs     string
 }
 
 func DefaultConfig() Config {
@@ -29,9 +30,10 @@ func DefaultConfig() Config {
 		MemMiB:      envOrInt("FC_MEM_MIB", 128),
 		Bridge:      envOr("FC_BRIDGE", "fcbr0"),
 		BridgeCIDR:  envOr("FC_BRIDGE_CIDR", "172.30.0.1/24"),
-		GuestPrefix: envOr("FC_GUEST_PREFIX", "172.30.0"),
-		GuestGW:     envOr("FC_GUEST_GW", "172.30.0.1"),
-		BootArgs:    "console=ttyS0 reboot=k panic=1 pci=off i8042.noaux quiet loglevel=0",
+		GuestPrefix:  envOr("FC_GUEST_PREFIX", "172.30.0"),
+		GuestGW:      envOr("FC_GUEST_GW", "172.30.0.1"),
+		AutoNetConfig: envOr("FC_AUTO_NET_CONFIG", "") == "1",
+		BootArgs:     "console=ttyS0 reboot=k panic=1 pci=off i8042.noaux quiet loglevel=0",
 	}
 	c.Kernel = envOr("FC_KERNEL", c.BaseDir+"/vmlinux")
 	c.KernelURL = envOr("FC_KERNEL_URL",