spec: reconcile NHP header size and packet framing with CSA standard

Summary

The CSA NHP standard describes a fixed NHP header size of 160 bytes for the standard/international profile and 224 bytes for the extended/domestic profile, with the ciphertext body immediately following the header. It also describes message length as the encrypted body length, supporting UDP and TCP short-connection framing.

Current OpenNHP main appears to use a different wire layout:

• nhp/core/scheme/curve/header.go: HeaderSize = HeaderCommonSize + PublicKeySize + MaximumIdentitySize + GCMTagSize + PublicKeySize + GCMTagSize + TimestampSize + GCMTagSize + HashSize, which is 240 bytes for the Curve profile.
• nhp/core/scheme/gmsm/header.go: uses the same formula, also producing 240 bytes.
• nhp/core/constants.go: PacketBufferSize = 4096, creating a much smaller practical packet/body limit than the standard's UDP-size framing language.
• nhp/core/initiator.go: rejects bodies larger than PacketBufferSize - mad.header.Size().

Why This Matters

A strict standard implementation expecting 160/224-byte headers or larger standard-framed payloads will not interoperate cleanly with the current OpenNHP wire layout. If the current layout is intentional, it should be documented as an OpenNHP profile/extension rather than left ambiguous.

Suggested Acceptance Criteria

• Document a byte-level matrix for CSA standard header layout vs current OpenNHP Curve/GMSM layout.
• Decide whether OpenNHP should be strict wire-compatible with the CSA standard or advertise a distinct profile.
• If strict compatibility is required, add parser/assembler support and migration handling.
• Add golden tests for header size, type/length offsets, and packet/body size behavior.