Enable the parked line_descriptor LSDDetector wrapper #1971

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The managed LSDDetector wrapper (and its helper VectorOfKeyLine) have been parked behind #if false since they were first added — commit 2ea1ce87 ("add LSDDetector but commented it out"). It was never a removed feature: the groundwork is present and live.

What already exists

• Native: src/OpenCvSharpExtern/line_descriptor.cpp / line_descriptor.h (line_descriptor_LSDDetector_new1/new2/delete/detect1/detect2).
• Managed P/Invoke: NativeMethods_line_descriptor.cs — active (not disabled).
• The line_descriptor module is present in opencv_contrib.

What is disabled

• src/OpenCvSharp/Modules/line_descriptors/LSDDetector.cs — whole file #if false.
• src/OpenCvSharp/Internal/Vectors/VectorOfKeyLine.cs — whole file #if false (its only consumer is the disabled LSDDetector).

Tasks

• Remove the #if false from LSDDetector.cs and VectorOfKeyLine.cs; make them compile against current APIs.
• Verify KeyLine struct marshalling (layout vs cv::line_descriptor::KeyLine) — the most likely reason it was parked.
• Add a smoke test (detect lines on a simple image; assert non-empty / sane KeyLine fields).
• Confirm the native line_descriptor exports are actually built into OpenCvSharpExtern for the CI configurations.

Notes

• VectorOfKeyLine was modernized in Eliminate ArrayAddress1; make ArrayAddress2 a scoped ref struct #1970 (its old ArrayAddress1 + Buffer.MemoryCopy body is now ReadOnlySpan<T>(...).ToArray()), so a re-enabler does not need to touch that.
• Requires a native build; validate on CI.

Separate, optional feature work (not part of the #1941 cleanup).
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