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.

coverage/summary.json

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