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>

coverage/summary.json

@@ -1,18 +1,18 @@
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@@ -59,17 +59,17 @@
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