Extend example missions library

[vking1996]

Adds 4 new example mission JSON files following the existing MissionPlan schema:

• three_drone_v_formation.json — 3 drones in V formation, 2m square
• racetrack_two_drones.json — 2 drones with staggered start, independent legs
• search_and_rescue_grid.json — Lawnmower grid search, 10m x 10m area
• marina_perimeter_patrol.json — GPS outdoor patrol, 80m x 60m perimeter

All pass schema validation and run in dryrun.

Closes #3