Headers

• message.h
  Defines the core message model. etl::imessage is the base interface (virtual or non-virtual, depending on ETL_HAS_VIRTUAL_MESSAGES). etl::message<ID> provides typed messages with static IDs. Type traits (is_message, is_message_type, etc.) and ID comparison utilities support compile-time validation.
• message_router.h
  Defines etl::imessage_router, the central routing interface with receive, accepts, and router identity. Provides message_router<TDerived, …> that statically dispatches by message ID (contiguous IDs optimized). Includes null_message_router and message_producer helpers, plus send_message utilities.
• message_bus.h
  Implements etl::imessage_bus, a router that manages a sorted list of subscribed routers and forwards messages based on destination ID (broadcast or addressed). Supports subscription limits and forwards to successors.
• message_broker.h
  Implements etl::message_broker, a router with explicit subscription objects mapping routers to message ID lists (span). It routes only to subscribers that match both message ID and destination ID, then forwards to any successor.
• message_packet.h
  A type-erased, in-place container for a fixed set of message types. Validates message ID acceptance, supports copy/move, and exposes get() as etl::imessage&. Uses aligned storage sized to the largest message type.
• shared_message.h
  Reference-counted wrapper around pooled messages (ireference_counted_message_pool). Supports copy/move semantics with automatic release when the last reference drops.

Basic architecture