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/package.json

@@ -0,0 +1,23 @@
+{
+  "name": "@tts/engine",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "import": "./dist/index.js"
+    }
+  },
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "typecheck": "tsc -p tsconfig.json --noEmit",
+    "test": "vitest run",
+    "lint": "echo \"no lint configured\""
+  },
+  "devDependencies": {
+    "typescript": "^5.7.2"
+  }
+}