4.2.0

AstronomyKit 4.2.0 fixes numerical rates and light-time calculations near discontinuities in the Delta T model and adds an API for short Terrestrial Time offsets. It includes the changes from #3, #4, and #6.

• Added AstroTime.addingTerrestrialDays(_:), which offsets both UT and TT by the supplied number of days. Use it for short numerical-derivative offsets; continue using addingDays(_:) for civil-time steps.
• Rate stencils and light-time iterations now use continuous TT offsets. This fixes errors near historical Espenak–Meeus model joins, where UT-based offsets could introduce a model step into a derivative or light-time correction.
• AstroTime(tt:) now preserves the supplied finite TT exactly, including values inside a forward Delta T step. Its derived UT remains an approximation where the model has no exact inverse.
• Refreshed the yearly observed Delta T table against IERS C04 through September 27, 2026 and Bulletin A predictions retrieved October 27, 2026. The entries remain unchanged at the stored millisecond precision and still extend through Julian epoch 2027.0.
• AstronomyConfig.ephemerisVersion is now 4.2.0. Calculated results can change from 4.1.0; provenance records and numerical caches should use this identifier.

This is a minor release because it adds public API. The package's approximately one-arcminute accuracy contract is unchanged.

Validation: strict Swift formatting and package-size checks passed. All 631 tests passed in debug and release configurations on macOS (Swift 6.4) and Linux (Swift 6.2), including the JPL and audit accuracy suites. All 631 tests also passed under ThreadSanitizer on macOS.

Full comparison: 4.1.0...4.2.0