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// engine/audio/AudioMixer.cpp
auto AudioMixer::process(std::span<const float> input)
    -> std::expected<Buffer, AudioError>
{
    auto mixed = m_pool.allocate(input.size());
    if (!mixed)
        return std::unexpected(AudioError::OutOfMemory);

    std::ranges::transform(input, mixed->begin(),
        [this](float s) { return s * m_gain; });

    return *mixed;
}
// C++23. std::expected. Zero heap allocation. RAII.