Add time scale conversion functions and TempochScale enum

[VPRamon]

This pull request adds comprehensive support for time scale conversions in the FFI layer, enabling conversions between Julian Date (TT) and a wide range of standard astronomical and civil time scales. It introduces both individual conversion functions and a generic dispatch mechanism, along with an enumeration for time scales to facilitate these operations from C/C++ code.

Key changes:

Time Scale Conversion Functions:

• Added FFI functions for converting Julian Date (TT) to and from the following time scales: TDB, TT, TAI, TCG, TCB, GPS, UT, JDE, and Unix Time. Each function is exposed via extern "C" for use in C/C++ environments. [1] [2]

Generic Conversion Dispatch:

• Introduced tempoch_jd_to_scale and tempoch_scale_to_jd functions, which allow conversion between Julian Date and any supported time scale using a new TempochScale enum as a parameter. This makes it easy to perform conversions when the target scale is not known at compile time. [1] [2]

Time Scale Enumeration:

• Defined a new TempochScale enum in both C and Rust, listing all supported time scales with explicit integer values for safe interop. This is used by the generic conversion functions for dispatch. [1] [2]

ΔT Calculation:

• Added tempoch_delta_t_seconds, an FFI function to compute ΔT (TT − UT1) in seconds for a given Julian Date, using the model from tempoch-core. [1] [2]

Internal Improvements:

• Updated Rust imports to include all relevant time scale types, ensuring the new conversion functions are available and type-safe.

These changes significantly enhance the flexibility and completeness of the time scale conversion API, making it much easier to interoperate with various time standards in cross-language contexts.

[github-actions]

Package	Line Rate	Complexity	Health
tempoch-core.src	99%	0	✔
tempoch-ffi.src	67%	0	➖
Summary	91% (1629 / 1791)	0	✔