feat(core-consent): add extractAnonymousConsent and migrateAnonymousConsent helpers

Migration helpers let the auth signUp flow transfer anonymous cookie consent
to an authenticated user's record. extractAnonymousConsent parses the raw
Cookie header; migrateAnonymousConsent calls IConsent.grant with
method: "signup-migration" for each granted category, making the migration
traceable in the audit log. No-op when the consent cookie is absent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-consent/src/index.ts

@@ -16,3 +16,8 @@ export {
   type ConsentFactory,
 } from "./di/bind-production";
 export { bindDevSeedConsent } from "./di/bind-dev-seed";
+export {
+  CONSENT_COOKIE_NAME,
+  extractAnonymousConsent,
+  migrateAnonymousConsent,
+} from "./migration";