Implement relay.hpp header (C++ reference, spec §18.2)

[SoundMatt]

RELAY spec v1.3 (release) completes the C++ binding in §18.2. This issue tracks building the header-only relay.hpp against that binding.

Scope

A header-only relay.hpp exposing the relay namespace (and nested relay::{can,dds,lin,mqtt,rcp,someip}):

• Core: Protocol, Version, Message, Errc (the four sentinels, registered as std::error_category "relay"), Context, Channel<T>, SubscriberOptions, Node/Caller, kRelaySpecVersion.
• Canonical types per §18.2 with fixed-width <cstdint> fields and §15 JSON field names; CAN XL fields included.
• to_message() member / from_message() free function (C++17 std::pair form; C++23 std::expected form guarded by __cpp_lib_expected).
• Validators (can::validate_frame, lin::validate_frame, someip::validate, …).
• Round-trip tests against spec/vectors/ and schema checks against spec/schemas/.

Notes

• Where cpp-RCP already provides relay::Context/relay::Channel aliases, reuse them (§18.2).
• Out of scope: per-protocol wire/transport implementations (those live in the respective x-Net C++ projects).