Replace the CommandHost with a handler registry keyed by message type.
Message is a permissive base type the host extends; CommandResult is a
discriminated union on the type suffix. Commands and orchestrators are
the same async shape taking a RunContext with an AbortSignal; runCommand
emits :done/:cancel/:error.
New @tts/engine package: the pure message layer that drives bgm games.
MessageQueue snapshots and drains on tick; TriggerRegistry matches by
type/id with runtime enable/disable; runOrchestrator suspends on wait
until a matching message is processed. Engine ties them together behind
the CommandHost contract that @tts/tabletop will implement.
hypercross