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-tabletop.md

@@ -53,7 +53,9 @@ the format's stacking strategy (`curve` / `limit` / `align` / `steps` / `tilt` /
 
 Scripted interaction — focus, tap-to-advance, move, caption — is built on an
 async command layer. See [`bgm-commands.md`](./bgm-commands.md) for command
-execution (lifecycle, run contexts, tap interaction).
+execution (lifecycle, run contexts, tap interaction), and
+[`bgm-engine.md`](./bgm-engine.md) for the message layer above it (the queue,
+triggers, and orchestrators that declare and fire commands).
 
 ## 6. usage