Add a Spanning Tree page: an on/off toggle driving the existing "stp"
command over /cmd, and a live status section fed by a new /stp.json
endpoint - the elected root bridge (priority + MAC), our path cost,
whether we are the root, and the per-port STP state read live from the
ASIC's MSTP register (same 2-bit encoding stp_setup() writes). Ports are
reported by their physical numbers.
Recovered-from: 3132319, 9365c86
