Document a deterministic packet-encoding test vector

[rufatixx]

Goal

Create one deterministic, human-reviewable test vector for the authenticated packet encoding used by RelayPacketCodec.

Deliverables

• fixed values for every authenticated metadata field
• the exact encoded byte sequence in hexadecimal
• a field-by-field explanation of byte ordering and string encoding
• a test that fails if the encoding changes unintentionally
• a note explaining that encryption nonces and ephemeral keys remain random

Why

A stable vector makes compatibility review easier before another implementation or physical transport is attempted.