v0.2.0

Full Conversion Matrix & Typed Conversion API

This release introduces a complete 6×6 GNSS time conversion matrix,
a unified conversion API, and improved correctness around leap seconds.

Highlights:

• Full conversion matrix (30 directions)
• Runtime conversion graph inspection (ConversionMatrix)
• Typed conversion traits: IntoScale, IntoScaleWith
• Leap second–aware conversions with ambiguity handling
• GLONASS-specific time utilities
• <1 ns roundtrip accuracy (except intrinsic leap second ambiguity)

This is a foundational release: gnss-time is now a complete,
type-safe core for GNSS time transformations.