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/architecture.md

@@ -32,6 +32,7 @@ shared types/validation package.
 | `packages/tts`     | Fetch save from Steam, BSON-parse to `TTSMod` | Node       |
 | `packages/extract` | Analyze a `TTSMod`: objects, refs, assets   | Isomorphic   |
 | `packages/mesh`    | 2D shapes + extrusion into 3D mesh geometry | Isomorphic   |
+| `packages/engine`  | Message bus, queue/tick, triggers, orchestrators | Isomorphic   |
 | `packages/shared`  | Shared types + zod schemas                  | Isomorphic   |
 
 ## Dependency graph