spec: align encrypted timestamp units with CSA NHP standard

Summary

The CSA NHP standard describes the encrypted timestamp field as an 8-byte UNIX timestamp in milliseconds since epoch plus a 16-byte AEAD tag.

Current OpenNHP main serializes nanoseconds:

• nhp/core/initiator.go: mad.LocalInitTime = time.Now().UnixNano().
• nhp/core/initiator.go: binary.BigEndian.PutUint64(tsBytes[:], uint64(mad.LocalInitTime)) writes that nanosecond value into the encrypted timestamp field.
• Receiver-side replay/staleness logic also operates on nanosecond values.

Why This Matters

OpenNHP peers agree with each other today, but a strict CSA-standard peer using millisecond timestamps will not interoperate correctly. Because the timestamp feeds freshness/replay checks, unit drift is security-sensitive and should be explicit.

Suggested Acceptance Criteria

• Decide whether the wire timestamp should be milliseconds per CSA standard.
• If yes, add versioned/backward-compatible handling for existing nanosecond-format peers.
• Add encoding KATs for timestamp units.
• Add replay/staleness tests covering standard-compatible packets and any compatibility mode.