:triangular_flag_on_post: Gate python bindings behind pyo3 feature flag
More clearly separating pure Rust parts from the pyo3/python bindings to workaround some pyo3 compiliation issues on rustc 1.92.0-beta.1 (3b4dd9bf1 2025-10-28), plus it speeds up compilation when just working on the Rust side.
Re-enabled nightly Rust tests disabled in 939b7b0c8f3e465b45a71cf5c0ff460667bc6f4e/#57.
:goal_net: Remove unwrap, use map_err instead
Bump nvtiff-sys from 0.1.2 to 0.1.3 to get Error trait on NvtiffStatusError, and then we can cast to string and pass error message to PyValueError.
:goal_net: Remove some unwraps and improve unimplemented docs
Raise NvTiffStatusError where possible, or a slightly helpful message for unimplemented branches. Document that f16 and complex dtypes and certain compression types may not be supported yet.