Messages are a discriminated union with generic command/result types;
the command host is dropped for a handler registry; commands and
orchestrators are the same async shape taking a cancellable RunContext.
Define the message/queue/tick model, the three handler kinds (trigger,
orchestrator, command host), and the @tts/engine package split. Cross-link
from bgm-commands and bgm-tabletop, and add the engine to the architecture
package table.
hypercross