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.
Add bgm-commands.md covering async commands with ok/cancel/error
results, per-invocation run contexts, fire-and-forget vs self-managed
waiting, and tap interaction with part-local trigger points. Link it
from the tabletop design and plan docs, and record the decisions in
decisions.md.
hypercross