fix: export generated C/C++ ABI symbols under hidden visibility

[owenthcarey]

Summary

Generated C and C++ headers now tag every exported prototype with a portable WEAVEFFI_API visibility macro, so a non-Rust backend that implements the header exports its symbols even under hidden default visibility (-fvisibility=hidden, the release-build norm and the MSVC default). Consumers resolve it to __declspec(dllimport) on Windows; it's benign elsewhere. Deprecated functions use a companion WEAVEFFI_DEPRECATED macro that also compiles under MSVC.

Changes

• Tag all C ABI prototypes (runtime helpers, functions, struct/enum ops) via the shared cabi renderer; emit the macro definitions from the C and C++ generators.
• Replace the bare __attribute__((deprecated)) with a portable WEAVEFFI_DEPRECATED macro.
• Add a producer-side conformance lane that builds an implementing C library under -fvisibility=hidden and asserts symbols export.
• Regenerate C/C++ snapshots; document the macro in the C generator docs and FAQ.

Closes #23

[ariankordi]

Thanks for the fix! I will test later today and report any more issues.
JSYK I am making a fork soon that will also have a small OpenGL sample to show you how my rendering use case works, demonstrating how I use Emscripten.