Complete BuffDetection pimpl pattern

[Copilot]

BuffDetection had the header declarations and internal Impl::buff_detect() logic but was missing the pimpl boilerplate that every other module implements.

• Define BuffDetectionResult struct (wraps shared_ptr<RuneTracker>)
• Add Impl::initialize() stub matching the public API contract
• Add constructor, destructor, initialize() and auto_detect() wrappers

Pattern follows ArmorDetection and other modules:

BuffDetection::BuffDetection() noexcept
    : pimpl { std::make_unique<Impl>() } { }

BuffDetection::~BuffDetection() noexcept = default;

auto BuffDetection::auto_detect(const BuffDetectionFrame& frame) noexcept
    -> std::optional<BuffDetectionResult> {
    auto result = pimpl->buff_detect(frame);
    if (!result) {
        return std::nullopt;
    }
    return BuffDetectionResult { std::move(result) };
}

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