feat(core-shared): add PII and retention type primitives

Introduces PiiCategory, DataProcessingPurpose, RetentionTrigger,
RetentionAction, FieldPii, FieldRetention, PAYLOAD_AUTH_PII_DEFAULTS,
PurgeSchedule, and CollectionRetention in core-shared/payload/.
Augments payload's FieldCustom and CollectionCustom interfaces via
ambient declaration so downstream collection configs gain typed
custom.pii and custom.retention / custom.authPii fields.

Credential fields (password, salt, hash, resetPasswordToken,
resetPasswordExpiration, loginAttempts, lockUntil, apiKey, apiKeyIndex)
are null in PAYLOAD_AUTH_PII_DEFAULTS to exclude security material
from DPA mapping. Adds @vitest/coverage-v8 and coverage exclusions
for boilerplate infrastructure files so coverage:diff is gated on
new executable code.

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

coverage/summary.json

@@ -1,18 +1,18 @@
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