feat(engine): add message queue, triggers, and orchestrators

New @tts/engine package: the pure message layer that drives bgm games.
MessageQueue snapshots and drains on tick; TriggerRegistry matches by
type/id with runtime enable/disable; runOrchestrator suspends on wait
until a matching message is processed. Engine ties them together behind
the CommandHost contract that @tts/tabletop will implement.

packages/engine/tsconfig.json

@@ -0,0 +1,9 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "outDir": "./dist",
+    "rootDir": "./src",
+    "lib": ["ES2022", "DOM"]
+  },
+  "include": ["src"]
+}