feat: narrow generated types and validators to the real data contract

• RFC 3339 string formats (date, date-time, time, duration) now emit template-literal types plus matching regex validators. Previously only date-time was handled, as the opaque `Jsonify`.
• integer/int64 now maps to `bigint` (range exceeds Number.MAX_SAFE_INTEGER).
• enums emit a bare picklist/literal/union, including null and non-string members.

Breaking for consumers: `DateTime` is now a template-literal string instead of `Jsonify`, and int64 fields are `bigint` instead of `number`.

Also: CI runs on node 26; test target typechecks lib/ and bin/.