feat: RELAY spec v0.2 conformance — relay:: namespace, Channel<T>, Adapt()

[SoundMatt]

Summary

Implements RELAY spec §18.2 C++ bindings in cpp-RCP, closing issue #5.

• include/relay/relay.hpp: New relay:: namespace header with:

  • relay::Protocol enum (§3) — CAN=1…SOMEIP=6
  • relay::Message universal envelope (§4)
  • relay::Errc error category named "relay" with four mandatory sentinels: ErrClosed, ErrNotConnected, ErrTimeout, ErrPayloadTooLarge (§5.1)
  • relay::Context — lightweight deadline context (§18.2), now the canonical definition
  • relay::BackPressurePolicy + relay::SubscriberOptions with default depth 64 (§14, §18.2)
  • relay::Channel<T> — generic typed bounded channel; bool push() returns false when full or closed (§18.2)
  • relay::Node and relay::Caller abstract classes (§18.2)
  • relay::kRelaySpecVersion = "0.2" (§19.4)

• include/rcp/rcp.hpp: Updated:

  • rcp::Context → alias for relay::Context (§18.2)
  • rcp::StatusChannel → alias for relay::Channel<Status> (§18.2)
  • rcp_category()::equivalent() maps rcp::Errc → relay::Errc via std::error_condition equivalence (§5.2, §5.3)

• include/rcp/adapt.hpp: New file:

  • rcp::Adapt(shared_ptr<Controller>) → unique_ptr<relay::Caller> (§10.3)
  • zone_to_relay_id / zone_from_relay_id for PascalCase zone names (§4.2)
  • status_to_message / response_to_message / message_to_command (§15.7.5)
  • RcpCallerAdapter implementing all three relay::Node/Caller virtual methods

• tests/test_relay.cpp: 23 new conformance tests — all 27/27 tests pass locally.

Spec gaps raised on RELAY repo (issue #6)

8 issues filed on SoundMatt/RELAY covering: SpecVersion inconsistency (§17.12 vs §19.4), Channel<T> by-value in subscribe() signature, missing C++ Adapt() signature, missing std::error_condition equivalence example, rcp.healthy meta key absent from §4.3, missing C++ ToMessage()/FromMessage() signatures, ErrAlreadyExists wrapping ErrClosed semantically wrong, and CLI conformance undefined for header-only C++ libraries.

Test plan

• Local build with clang++ / C++17 — all 27/27 CTest cases pass
• rcp_relay conformance test — 51 assertions in 23 test cases
• CI: Ubuntu clang-14/gcc-12, macOS clang, Windows MSVC matrix