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/src/trigger.test.ts

@@ -0,0 +1,59 @@
+import { describe, expect, it } from 'vitest';
+import { TriggerRegistry, triggerMatches, type Trigger } from './trigger.js';
+
+const tap: Trigger = {
+  type: 'tap',
+  id: 'draw',
+  match: { part: 'carcassonne:tile#a', trigger: 'draw' },
+  emit: [{ type: 'move', data: { part: 'carcassonne:tile#a', to: '/grid/5/5' } }],
+};
+
+describe('triggerMatches', () => {
+  it('matches on type and every match param', () => {
+    expect(
+      triggerMatches(tap, { type: 'tap', data: { part: 'carcassonne:tile#a', trigger: 'draw' } }),
+    ).toBe(true);
+  });
+
+  it('rejects a different type', () => {
+    expect(triggerMatches(tap, { type: 'focus' })).toBe(false);
+  });
+
+  it('rejects a mismatched param', () => {
+    expect(
+      triggerMatches(tap, { type: 'tap', data: { part: 'carcassonne:tile#b', trigger: 'draw' } }),
+    ).toBe(false);
+  });
+
+  it('matches any message of the type when there is no match block', () => {
+    const any = { type: 'focus', emit: [] };
+    expect(triggerMatches(any, { type: 'focus', data: { path: '/deck' } })).toBe(true);
+  });
+});
+
+describe('TriggerRegistry', () => {
+  it('registers and matches enabled triggers', () => {
+    const reg = new TriggerRegistry();
+    reg.register(tap);
+    expect(reg.match({ type: 'tap', data: { part: 'carcassonne:tile#a', trigger: 'draw' } })).toEqual([
+      tap,
+    ]);
+  });
+
+  it('collision-checks duplicate type#id', () => {
+    const reg = new TriggerRegistry();
+    reg.register(tap);
+    expect(() => reg.register({ ...tap })).toThrow(/Duplicate trigger: tap#draw/);
+  });
+
+  it('disable/enable toggles a trigger at runtime', () => {
+    const reg = new TriggerRegistry();
+    reg.register(tap);
+    reg.disable('tap', 'draw');
+    expect(reg.match({ type: 'tap', data: { part: 'carcassonne:tile#a', trigger: 'draw' } })).toEqual([]);
+    reg.enable('tap', 'draw');
+    expect(reg.match({ type: 'tap', data: { part: 'carcassonne:tile#a', trigger: 'draw' } })).toEqual([
+      tap,
+    ]);
+  });
+});