Source build skipped due to platform-agnostic native artifact check

[holodorum]

Bug

When installing kson-lang from an sdist, the native library for the target platform may not be built, resulting in a runtime error:

  File "cffi/api.py", line 829, in _load_backend_lib
    raise OSError(msg)
OSError: cannot load library '.../site-packages/kson/libkson.so':
  libkson.so: cannot open shared object file: No such file or directory.

Root cause

The build backend checks whether native artifacts already exist before building from source (lib-python/build_backend.py:21-23):

native_files = ["kson.dll", "libkson.dylib", "libkson.so", "kson_api.h"]
artifacts_exist = any((src_kson_dir / f).exists() for f in native_files)

This check is platform-agnostic — any() returns True if any of the listed files exist, regardless of whether that file is usable on the current platform. If a stale artifact from a
different OS is present (e.g. libkson.dylib from macOS), the check passes, the Gradle build is skipped, and the actually needed native library is never compiled.

Fix

Make the check platform-aware: only verify the native artifact relevant to the current OS:

• Linux: libkson.so
• macOS: libkson.dylib
• Windows: kson.dll

2. Ensure cross-platform artifacts are not accidentally included in published sdists so stale binaries from one platform don't end up
   triggering the skip on another.