feat: add country to ContactField, drop placeholder countries, and make finalizeContact idempotent

- Extend `ContactField` union and `REPORTED_FIELDS`/`ADR_INDEX_FIELDS`/`VCARD_PROPERTY_FIELDS` to include `"country"` so RFC 9537 redactions targeting the country field and vCard `adr[6]` are now tracked in `redactedFields`
- Drop placeholder country values (`"N/A"`, `"REDACTED FOR PRIVACY"`, etc.) in `finalizeContact`, but only after confirming the value isn't a real country code or name (e.g. `"NA"` resolves to Namibia and is kept)
- Seed `redactedFields` and `redacted` from the contact's existing values at the start of `finalizeContact` so calling it twice on the same contact produces identical output (idempotent)
- Expand `PLACEHOLDER_VALUE_PATTERNS` to match bare dot sequences (`".."`) and whole-value-only phrases (`"Not Available"`, `"Not Published"`, `"Not Public"`, `"No Data"`) without risk of false-positives on substrings like `"No Data Corp"`
- Export `finalizeContact` and `isPrivacyContact` from the package entry point

README.md

@@ -571,6 +571,7 @@ interface DomainRecord {
       | "state"
       | "postalCode"
       | "poBox"
+      | "country"
     >; // fields that were redacted (and are therefore absent)
     privacyService?: boolean; // name/organization is a privacy/proxy service (text kept), not a redaction notice
   }>;