feat(companion): add reusable heartbeat loop utility

src/companion/index.ts

@@ -7,6 +7,7 @@ export {
   IOSCompanionClient,
   AndroidCompanionClient,
 } from './platformClients.js';
+export { CompanionHeartbeatLoop } from './heartbeatLoop.js';
 
 export type {
   CompanionRuntimeClientOptions,
@@ -44,3 +45,7 @@ export type {
   HeartbeatStatusInput,
   PlatformBootstrapResult,
 } from './platformClients.js';
+export type {
+  HeartbeatPublisher,
+  CompanionHeartbeatLoopOptions,
+} from './heartbeatLoop.js';