BUG: Remove redundant m_Threader shadow in MatchCardinalityImageToImageMetric

[hjmjohnson]

Remove MatchCardinalityImageToImageMetric's redundant m_Threader shadow. The base ImageToImageMetric constructor already initializes its protected m_Threader to MultiThreaderBase::New() — identical to the subclass's intended default — so the subclass redeclaration served no purpose.

The shadow and why it was 100% redundant

itkMatchCardinalityImageToImageMetric.h:189 declared (in private:):

MultiThreaderBase::Pointer m_Threader{ MultiThreaderBase::New() };

itkImageToImageMetric.h:632 declares (in protected:):

MultiThreaderBase::Pointer m_Threader{};

The base's in-class init produces a null pointer, but the base's constructor (itkImageToImageMetric.hxx) initializes the field in its member-initializer list:

: ...
, m_Threader(MultiThreaderBase::New())

So when a MatchCardinalityImageToImageMetric is constructed, the base constructor runs first and sets m_Threader to a fresh MultiThreaderBase::New(). Then the subclass's in-class init for its shadow runs and ALSO produces a fresh New(). Two threaders allocated; only the subclass's shadow is reachable via the subclass's GetMultiThreader(). The base's threader is orphaned.

The fix

1. Delete the m_Threader redeclaration in MatchCardinalityImageToImageMetric's private: section.
2. Qualify the GetMultiThreader() accessor with this-> so the lookup resolves to the inherited protected member: return this->m_Threader;.

No constructor change is needed because the base already initializes the field.

Net diff: 4 lines removed, 1 line modified.

Local verification

• pre-commit run --all-files passes.
• ninja ITKRegistrationCommonHeaderTest1 ITKRegistrationCommonHeaderTest2 builds clean.
• itkMatchCardinalityImageToImageMetricTest CTest: pre-existing baseline failure on this machine (missing Spots.png ExternalData fixture) — verified to fail identically on upstream/main HEAD without my change. Not a regression from this PR.

No new GTest was added because the shadow removal is byte-equivalent (base ctor produces the same New() pointer the subclass's shadow used to allocate). The single existing public accessor (GetMultiThreader) is exercised by the existing test driver.

[hjmjohnson]

@greptile review