refactor(c/sedona-proj): replace C dynamic loading with pure Rust using libloading

[Kontinuation]

Summary

Replace the C-based dlopen/dlsym dynamic PROJ loading (proj_dyn.c/proj_dyn.h) with a pure Rust implementation using the libloading crate. This eliminates ~250 lines of C code while maintaining the same ProjApi struct-of-function-pointers architecture used by all existing call sites.

Approach

The libloading crate provides safe, cross-platform dynamic library loading (dlopen/LoadLibrary). Each PROJ symbol is loaded via a load_fn! macro that:

1. Loads the symbol as a raw *const () pointer
2. Transmutes it to the expected function pointer type
3. Stores it in the existing ProjApi #[repr(C)] struct

The Library handle is stored as _lib: Option<Library> in the Rust ProjApi wrapper — Some when loaded from a shared library, None when using proj-sys. This ensures the library stays loaded for the lifetime of the function pointers.

Tests

Temporarily enabled python-wheels test and passed on all major platforms: https://github.com/apache/sedona-db/actions/runs/22518558723

Upcoming Changes

We'll implement c/sedona-gdal using similar approach.

[Copilot]

Pull request overview

This PR refactors sedona-proj’s dynamic PROJ loading by removing the C-based dlopen/dlsym shim and replacing it with a Rust-only loader built on libloading, while preserving the existing ProjApi function-pointer table consumed by the rest of the crate.

Changes:

• Removed the C dynamic loader (proj_dyn.c/proj_dyn.h) and the Rust FFI entrypoint that initialized it.
• Added a Rust dyn_load module that loads required PROJ symbols via libloading and returns (Library, ProjApi) to keep the library alive.
• Updated crate/workspace dependencies: removed cc build dependency and added libloading.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
c/sedona-proj/src/proj_dyn_bindgen.rs	Removes the FFI declaration for the old C initializer.
c/sedona-proj/src/proj_dyn.h	Deletes the C header for the old dynamic loader API.
c/sedona-proj/src/proj_dyn.c	Deletes the old C implementation of symbol loading and library lifetime management.
c/sedona-proj/src/proj.rs	Switches shared-library initialization to the new Rust loader and keeps the Library handle alive in ProjApi.
c/sedona-proj/src/lib.rs	Adds the new dyn_load module.
c/sedona-proj/src/dyn_load.rs	New Rust implementation of symbol loading using libloading.
c/sedona-proj/build.rs	Removes the build script that compiled the deleted C code.
c/sedona-proj/Cargo.toml	Drops cc build-dependency and adds libloading.
Cargo.toml	Adds libloading to workspace dependencies.
Cargo.lock	Locks libloading and updates sedona-proj dependency set accordingly.

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[paleolimbot]

Thank you!