STP/RSTP BPDU decoding (IEEE 802.1D/802.1w)

[EONRaider]

Target: 2.5.0 (Tier 3 — L2 discovery & tunneling) — depends on
the 802.3 length-field/LLC-detection issue in this same tier.

Context

STP (IEEE 802.1D) BPDUs ride classic 802.3 + LLC framing — DSAP/SSAP
0x42, no SNAP — with no EtherType at all. This issue is undecodable
until the LLC-detection issue in this tier lands; it's filed now so
the tier's full shape is visible, not as work to start early.

Deliverable

Add netprotocols.STP (layer 2, dispatched from the new llc.dsap
table at DSAP 0x42):

• Decode the Configuration BPDU (IEEE 802.1D §9.3.1, 35 bytes):
  Protocol ID, Protocol Version ID, BPDU Type, Flags, Root ID (Bridge
  ID: 2-byte priority + 6-byte MAC), Root Path Cost, Bridge ID, Port
  ID, Message Age, Max Age, Hello Time, Forward Delay — all in the
  fixed-width, no-TLV shape this library already handles well.
• RSTP (802.1w) reuses the same BPDU shape with BPDU Type 0x02 and
  one additional Version 1 Length byte (always 0 in practice) — decode
  both Configuration and RSTP BPDUs as one class, branching on BPDU Type the way ICMPv4/ICMPv6 branch on message type.
• Topology Change Notification BPDUs (Type 0x80) are 4 bytes total
  (Protocol ID, Version, Type) — trivial, decode as the same class's
  short-form case.

Acceptance criteria

• STP decodes Configuration, RSTP, and Topology Change
  Notification BPDUs; bytes(STP.decode(x)) == x.
• LLC DSAP 0x42 dispatches to STP via the table added by the
  LLC-detection issue.
• Fixture with a real captured BPDU (from a switch or a bridged
  lab topology) and truncation.
• CHANGELOG entry; full ladder green.

References

IEEE 802.1D §9.3, IEEE 802.1w (RSTP) · blocked by: the LLC-detection
issue, this tier (#173)

Part of #173