fix(audience): guard shutdown() against double-call

Add destroyed flag so calling shutdown() twice (e.g. React strict mode
useEffect cleanup) doesn't queue duplicate session_end or decrement
liveInstances below the real count.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

packages/audience/sdk/src/sdk.test.ts

@@ -655,6 +655,24 @@ describe('Audience', () => {
       expect(msg.properties.duration).toBe(5);
     });
 
+    it('is safe to call twice (React strict mode)', async () => {
+      const sdk = createSDK({ consent: 'full' });
+      sdk.shutdown();
+
+      await Promise.resolve();
+      await Promise.resolve();
+      fetchCalls.length = 0;
+
+      sdk.shutdown();
+      await Promise.resolve();
+      await Promise.resolve();
+
+      const sessionEnds = sentMessages().filter(
+        (m: any) => m.eventName === SESSION_END,
+      );
+      expect(sessionEnds).toHaveLength(0);
+    });
+
     it('does not emit session_end at none consent', async () => {
       const sdk = createSDK({ consent: 'none' });
       sdk.shutdown();

packages/audience/sdk/src/sdk.ts

@@ -64,6 +64,8 @@ export class Audience {
 
   private isFirstPage = true;
 
+  private destroyed = false;
+
   private constructor(config: AudienceConfig) {
     const {
       cookieDomain,
@@ -419,6 +421,8 @@ export class Audience {
    * unmounts or the player leaves.
    */
   shutdown(): void {
+    if (this.destroyed) return;
+    this.destroyed = true;
     if (!this.isTrackingDisabled()) this.trackSessionEnd();
     this.queue.destroy();
     Audience.liveInstances = Math.max(0, Audience.liveInstances - 1);