NTP protocol support (RFC 5905)

[EONRaider]

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

Context

NTP synchronizes clocks, UDP port 123. Fixed-width header, no
options, no variants worth branching on for the common case — one of
the simplest protocols in this whole roadmap, a good first PR to open
the tier.

Deliverable

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

• Decode the 48-byte NTPv3/v4 header (RFC 5905 §7.3): LI/VN/Mode (1
  byte, three bitfields), Stratum, Poll, Precision (1 byte each),
  Root Delay, Root Dispersion, Reference ID (4 bytes each), Reference
  Timestamp, Origin Timestamp, Receive Timestamp, Transmit Timestamp
  (8-byte NTP short/long format each).
• NTP timestamps are seconds-since-1900 fixed point, not Unix epoch —
  add a display/accessor property that converts to a Python datetime
  or Unix-epoch float (mirroring how this library already surfaces
  hex/enum display helpers alongside raw fields), without silently
  replacing the raw on-wire value.
• Register udp.port 123 → NTP, best-effort like every other
  port-table entry.

Acceptance criteria

• NTP decodes the full header including all four timestamps;
  bytes(NTP.decode(x)) == x.
• A timestamp accessor converts NTP epoch to a usable Python value
  correctly (verify against a known reference timestamp).
• UDP port 123 dispatches to NTP.
• Fixture; truncation case.
• CHANGELOG entry; full ladder green.

References

RFC 5905

Part of #174