refactor(audience): move ImmutableWebSDK into sdk package

Move sdk.ts, sdk.test.ts, config constants, and WebSDKConfig into
@imtbl/audience-sdk. Flatten WebSDKConfig (no longer extends a base).
All imports now local — no cross-package dependency on @imtbl/audience-web-sdk.

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

Author: ImmutableJeffrey

packages/audience/sdk/src/config.ts

@@ -2,5 +2,19 @@
 // Backend endpoints and base URLs come from @imtbl/audience-core.
 
 export const LIBRARY_NAME = '@imtbl/audience-sdk';
-// Replaced at build time by esbuild replace plugin
+/** Replaced at build time by esbuild replace plugin. */
 export const LIBRARY_VERSION = '__SDK_VERSION__';
+
+/** Log prefix for console messages from this package. */
+export const LOG_PREFIX = '[audience-sdk]';
+
+/** Default consent source when consentSource is not provided in config. */
+export const DEFAULT_CONSENT_SOURCE = 'WebSDK';
+
+// --- Auto-tracked event names ---
+// These are fired by the SDK lifecycle, not by studio code.
+
+/** Fired on init (or consent upgrade from none) when no active session cookie exists. */
+export const SESSION_START = 'session_start';
+/** Fired on explicit shutdown(). Not fired on tab close or consent revocation. */
+export const SESSION_END = 'session_end';

packages/audience/sdk/src/index.ts

@@ -1,2 +1,4 @@
-export type { AudienceSDKConfig } from './types';
+export { ImmutableWebSDK } from './sdk';
+export type { WebSDKConfig } from './types';
 export { DebugLogger } from './debug';
+export type { Environment, ConsentLevel, UserTraits } from '@imtbl/audience-core';