Introduces the ExtensionData namespace with the full contract and service
layer that future extension records (municipality profiles, election jobs,
service configs) will use to store and validate their legacy foreign references.

- LegacyLinkType enum (JurisdictionJCode, KitId, ContactId)
- LegacyLinkReference value record with typed factory methods
- ILegacyLinkValidator / LegacyLinkValidator — validates each link type
  against the anti-corruption layer; returns descriptive errors on failure
- ILegacyLinkedRecord / ILegacyLinkedRecordProvider — contracts extension
  record types implement to participate in integrity checks
- ILegacyLinkIntegrityCheck / LegacyLinkIntegrityService — scans all
  registered providers, computes consistency percentage (NFR13 target 99.9%)
- POST /api/admin/legacy-link/integrity-check (Admin-only, audit-logged)
- 19 unit tests covering all four ACs

Stories 1.10-1.13 register ILegacyLinkedRecordProvider implementations.

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