Adds draw_home_marker() in overlay.py (red cross at projected home
position) and draw_airspace() in airspace.py (GeoJSON Polygon
boundaries in blue, graceful FileNotFoundError handling). Includes
airspace fixture and three tests covering all acceptance criteria.
All 70 tests pass; ruff check and format clean.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>