firecracker: adopt firecracker-jailer for VM launch (Phase B)

Each VM's firecracker now runs inside a per-VM chroot dropped to the
registered owner UID via firecracker-jailer. Closes the broad ambient-
sudo escalation surface that survived Phase A: the helper still needs
caps for tap/bridge/dm/loop/iptables, but the VMM itself no longer
runs as root in the host root filesystem.

The host helper stages each chroot up front: hard-links the kernel
and (optional) initrd, mknods block-device drives + /dev/vhost-vsock,
copies in the firecracker binary (jailer opens it O_RDWR so a ro bind
fails with EROFS), and bind-mounts /usr/lib + /lib trees read-only so
the dynamic linker can resolve. Self-binds the chroot first so the
findmnt-guarded cleanup can recurse safely.

AF_UNIX sun_path is 108 bytes; the chroot path easily blows past that.
Daemon-side launch pre-symlinks the short request socket path to the
long chroot socket before Machine.Start so the SDK's poll/connect
sees the short path while the kernel resolves to the chroot socket.
--new-pid-ns is intentionally disabled — jailer's PID-namespace fork
makes the SDK see the parent exit and tear the API socket down too
early.

CapabilityBoundingSet for the helper expands to add CAP_FOWNER,
CAP_KILL, CAP_MKNOD, CAP_SETGID, CAP_SETUID, CAP_SYS_CHROOT alongside
the existing CAP_CHOWN/CAP_DAC_OVERRIDE/CAP_NET_ADMIN/CAP_NET_RAW/
CAP_SYS_ADMIN.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/model/types.go

@@ -22,6 +22,7 @@ const (
 	DefaultStatsPollInterval        = 10 * time.Second
 	DefaultStaleSweepInterval       = 1 * time.Minute
 	MaxDiskBytes              int64 = 128 * 1024 * 1024 * 1024
+	DefaultJailerBinary             = "/usr/bin/jailer"
 )
 
 type VMState string
@@ -36,6 +37,9 @@ const (
 type DaemonConfig struct {
 	LogLevel           string
 	FirecrackerBin     string
+	JailerBin          string
+	JailerEnabled      bool
+	JailerChrootBase   string
 	SSHKeyPath         string
 	HostHomeDir        string
 	AutoStopStaleAfter time.Duration