v0.1.0 of vss_crash_event

Pure-Dart typed data classes for vehicle crash/impact events, modelled on COVESA Vehicle Signal Specification (VSS) vocabulary. JSON round-trip, zero rendering dependencies. Changelog excerpt: - Initial release. - Typed data classes modelled on COVESA VSS vocabulary: CrashEvent, ObstacleDetection, AirbagStatus, BrakeAssistStatus. - ObstacleWarningTypeenum mirroring VSS Vehicle.ADAS.ObstacleDetection.WarningTypeallowed values. - Derived CrashSeverityclassification via CrashEvent.classifySeverity. - JSON round-trip (toJson/ fromJson) for all types. - Boundary validation of VSS unit constraints (non-negative distance/time gap).