Add comprehensive haplogroup discovery system design document

- Introduced a detailed `haplogroup-discovery-system.md` document outlining a unified approach for evolving Y-DNA and mtDNA haplogroup trees based on biosample discoveries.
- Addressed gaps in tracking private variants, consensus formation, curator workflows, and tree evolution.
- Included architectural overview, data models, schema reorganization, and service layer definitions to support the system.

Author: JamesKane