Replace slice-based aircraft tracking with map[string]bool

[Copilot]

The spotted-aircraft state was managed as a []Aircraft slice, requiring four helper functions (containsAircraft, newlySpotted, updateSpottedAircraft, validateAircraft) and awkward pointer-to-slice threading across the call stack to implement what is fundamentally a set membership check.

Changes

• geebee.go: Removed the four slice-management helpers. HandleAircraft now accepts a map[string]bool (keyed by ICAO) and inlines the logic — O(1) lookup/insert for new sightings, a single range+delete pass to evict aircraft that left range.
• main.go: Replaced *[]geebee.Aircraft with map[string]bool{} — no pointer indirection needed since maps are reference types.

// Before: 4 functions + *[]Aircraft threading
func HandleAircraft(alreadySpottedAircraft *[]Aircraft) ([]AircraftOutput, error)

// After: map passed directly, logic inlined
func HandleAircraft(spotted map[string]bool) ([]AircraftOutput, error)

Net: −48 lines, +21 lines, identical behaviour.