🚀 Feature

Add an optional config map parameter to metacall_load_from_X functions so callers can pass explicit compiler flags, include paths, and library paths directly — replacing the current fragile path-guessing approach.

Is your feature request related to a problem?

metacall_load_from_package and related functions have no way to accept loader-specific configuration. The C loader works around this by guessing paths from execution_paths, which breaks whenever headers and libraries are in different directories. There is also no way to pass compiler flags (-std=c++17, -DFOO, etc.) or extra include/library paths at call time.

This was directly exposed by #631 where load_from_package failed to find libloadtest.so and loadtest.h because they were in different locations than the loader expected.

Describe the solution you'd like

Add an optional void *config parameter (a metacall_value map) to the load functions:

// Current
int metacall_load_from_package(const char *tag, const char *name, void **handle);

// Proposed
int metacall_load_from_package(const char *tag, const char *name, void **handle, void *config);

The config map is loader-defined. For the C loader, supported keys would be:

Passing NULL preserves the current behavior for backward compatibility.

Files to change: metacall_loaders.h, metacall_loaders.c, the loader interface in source/loader/, and c_loader_impl.cpp to consume the keys. Other loaders can add their own keys incrementally.

Describe alternatives you've considered

• Keep using execution_paths guessing — already proven fragile (metacall-c-lib-test fails: C loader load_from_package cannot reliably find .so and .h via execution_paths #631). Not scalable.
• Environment variables (LOADER_LIBRARY_PATH, etc.) — global and not per-call. Can't pass per-invocation flags.
• Separate config API call before load — adds state and ordering requirements, less clean than a single call.

Additional context

This was discussed with @viferga as the next planned contribution after the immediate fix in #631. The config map approach is already part of the broader design direction for the metacall_load_from_X API.