feat(audience): add ImmutableWebSDK with consent, tracking, and identity

ImmutableWebSDK.init() provides consent-aware event tracking for web:
- track(), page(), identify(), alias() with three-tier consent gating
- Uses core's createConsentManager (shared with pixel), collectAttribution,
  and getOrCreateSession — no duplicated logic
- session_start/session_end lifecycle with duration tracking
- Attribution capture (UTM, click IDs, referrer) on first page view
- setConsent() with queue cleanup on downgrade, session restart on upgrade
- reset() for player logout, flush() for manual send, shutdown() for cleanup

46 tests covering all public methods, consent transitions, and edge cases.

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

Author: ImmutableJeffrey

packages/audience/sdk/src/index.ts

@@ -1,3 +1,3 @@
-export type { AudienceSDKConfig } from './types';
-export { DebugLogger } from './debug';
-export { collectContext } from './context';
+export { ImmutableWebSDK } from './sdk';
+export type { WebSDKConfig } from './types';
+export type { Environment, ConsentLevel, UserTraits } from '@imtbl/audience-core';