feat: add ImmutableWebSDK class

init, page, track, identify (known + anonymous), alias, setConsent,
reset, flush, shutdown. Consent-aware gating. Server reconciliation on init.

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

Author: ImmutableJeffrey

packages/audience/web/src/index.ts

@@ -0,0 +1,14 @@
+// Main SDK class
+export { ImmutableWebSDK } from './sdk';
+
+// Events
+export { AudienceEvent, IdentityProvider } from './events';
+export type { EventParamMap, Identity } from './events';
+
+// Types
+export type {
+  WebSDKConfig,
+  ConsentLevel,
+  UserTraits,
+  Environment,
+} from './types';