The `LoadedNativeLibrary` interface just re-listed the sixteen fields of
the object literal `loadBindings` already built, one liability the
methods below never needed a second time. Split the library-building
body into `buildLibrary()` and let `LoadedNativeLibrary` be
`ReturnType<typeof buildLibrary>`; `loadBindings` is now a thin cache
wrapper around it. `nextEventAsync`'s signature stays explicit as
`NextEventAsyncFn`, since `promisify` can't infer one from koffi's
untyped `.async`.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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