docs: add bgm-engine message layer design

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.

docs/bgm-commands.md

@@ -6,8 +6,10 @@ interaction — focus the camera, wait for a tap, move a part, show a caption
 that runs against the tabletop state store and render layer.
 
 This doc covers **command execution**: the async lifecycle, run contexts, and
-tap interaction. How commands are *declared* (the `script` role, trigger
-points on parts) is a separate concern, deferred to `bgm-format.md`.
+tap interaction. The message layer above this — how commands are *declared*
+and *fired* (triggers, orchestrators, the message queue) — is specified in
+[`bgm-engine.md`](./bgm-engine.md). The command host is the `@tts/tabletop`
+implementation of the engine's `CommandHost` contract.
 
 ## 1. async commands