Fix encounter data: sub-areas and add validation tests

- Create separate location areas for each sub-area (Route 2 southern/northern,
  Mt. Moon 1F/B1F/B2F, Rock Tunnel, Seafoam Islands, Victory Road, Cerulean Cave)
- Deduplicate encounters that appeared across multiple sub-areas
- Add pytest validation suite (tests/csv_validation/) that checks:
  - Foreign key integrity (slots, location areas, methods)
  - Pokemon IDs valid for the game
  - Encounter methods valid for the game
  - Level ranges within bounds
  - Version exclusives correctness
  - No duplicate encounter entries
- Tests are game-agnostic and parametrized - add a GameConfig to support
  additional games (BDSP, SwSh, etc.)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: DJ-Meyers