HSRP protocol support (RFC 2281)

[EONRaider]

Target: 2.6.0 (Tier 4 — AAA & infrastructure services)

Context

HSRP is Cisco's router-redundancy protocol, UDP port 1985 — the same
role VRRP fills (Tier 2, this roadmap), and close enough in shape
that it's worth doing after VRRP ships, so both can share test-fixture
conventions.

Deliverable

Add netprotocols.HSRP (layer 7, dispatched from udp.port):

• Decode the 20-byte HSRPv1 header (RFC 2281 §5.1 — HSRP is a Cisco
  protocol documented only informationally): Version (1 byte), Op Code
  (1 byte), State (1 byte), Hellotime (1 byte), Holdtime (1 byte),
  Priority (1 byte), Group (1 byte), Reserved (1 byte), Authentication
  Data (8 bytes, plaintext — no cryptographic meaning, decode as-is),
  Virtual IP Address (4 bytes).
• Dispatch on Op Code: Hello (0), Coup (1), Resign (2) — all share
  the same fixed header shape, so this is a field-value distinction,
  not a structural branch like VRRP's version differences.
• Register udp.port 1985 → HSRP, best-effort like every other
  port-table entry.

Acceptance criteria

• HSRP decodes the fixed header for all three Op Codes;
  bytes(HSRP.decode(x)) == x.
• UDP port 1985 dispatches to HSRP.
• Fixture covering Hello and at least one of Coup/Resign;
  truncation.
• CHANGELOG entry; full ladder green.

References

RFC 2281 (informational) · Router-redundancy sibling: VRRP (#172)

Part of #174