RADIUS protocol support (RFC 2865)

[EONRaider]

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

Context

RADIUS is the dominant AAA protocol, UDP ports 1812/1813
(authentication/accounting; legacy 1645/1646 still seen). Fixed
header plus a flat TLV attribute list — no nesting, the simplest of
this tier's two TLV-based protocols (the other is STUN, this same
tier).

Deliverable

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

• Decode the 20-byte header (RFC 2865 §3): Code (1 byte), Identifier
  (1 byte), Length (2 bytes), Authenticator (16 bytes, opaque —
  computing/verifying it requires the shared secret, out of scope).
• Decode the Attribute list (Type 1 byte, Length 1 byte including the
  2-byte TLV header, Value) via _tlv.py's walk_kind_length_value.
• Name the most common attribute types: User-Name (1), User-Password
  (2, keep raw — it's obfuscated with the shared secret, not
  decodable without it), NAS-IP-Address (4), NAS-Port (5),
  Framed-IP-Address (8), Vendor-Specific (26, keep raw — vendor TLVs
  nested inside are a follow-up, not this issue).
• Register udp.port 1812 and 1813 → RADIUS, best-effort like every
  other port-table entry.

Acceptance criteria

• RADIUS decodes the header and attribute list;
  bytes(RADIUS.decode(x)) == x.
• Named attribute types decode to typed fields; others remain
  correctly shaped but raw.
• UDP ports 1812/1813 dispatch to RADIUS.
• Fixture covering an Access-Request/Access-Accept pair;
  truncation.
• CHANGELOG entry; full ladder green.

References

RFC 2865 · TLV walker: _tlv.py

Part of #174