feat(core-shared): add subject-linkage types and extend PII defaults

Introduces SubjectLinkKind, SubjectLink, and CollectionSubject types to
packages/core-shared/src/payload/subject-linkage-types.ts, establishes the
ambient CollectionCustom.subject declaration (parallel to custom.pii / custom.retention
from Epic A), and extends PAYLOAD_AUTH_PII_DEFAULTS with processingRestrictedAt and
consentState as DSR-managed excluded fields. Applies the first canonical usage of
custom.subject = { kind: "self", field: "id" } on the auth users collection.

packages/auth/src/integrations/cms/collections/users.ts

@@ -15,6 +15,7 @@ export const users: CollectionConfig = {
         action: "hard-delete",
       },
     },
+    subject: { kind: "self", field: "id" },
   },
   fields: [
     {