Add ground-station-based satellite availability generation

[Nova-David]

• Adds generate_ground_stations.py: places N ground stations (default 8) at uniform-random points on the globe.
• Adds sat_availability.py: computes real elevation-angle geometry (from each satellite's ECEF position and each ground station's coordinates) to determine, per satellite per timestep, whether it's above a station's horizon.
• A satellite counts as available only once it's stayed visible for a configurable minimum window (90s default), not from the moment it's geometrically visible, since a round needs time to actually complete.
• Each ground station's visibility is truncated to that minimum window independently, before being combined across stations (|=), not on the union of raw visibility. Prevents crediting a satellite with time that would require an impossible handoff between two physically separate stations.
• Output is a per-satellite (timestamp, state) trace in the same AVL_TRAIN/UN_AVL format as the existing syn_0/syn_20 traces, keyed by trainer_###, so it's a drop-in replacement by format.