Avoid re-reading attributes when copying DxfProperty

[s1lentcs]

Description

Reading DXF files with many VERTEX entities spends most of its time in reflection. DxfSectionReaderBase.readVertex calls DxfClassMap.Create<T>() for every vertex; Create returns a copy of the cached map, and the DxfProperty copy constructor chains to DxfPropertyBase(PropertyInfo), which calls GetCustomAttribute<T>() again for every property. The attribute data is already available on the source property, so it is re-read needlessly once per property per vertex.

This PR adds a protected copy constructor to DxfPropertyBase<T> that reuses the source's _attributeData, _property and _assignedCode, and routes the DxfProperty copy constructor through it.

Results (large real-world DXF made up mostly of 3D polylines, Release build, runs interleaved over several rounds):

	master	master + this PR	Change
DxfReader.Read (warm, median)	7,093 ms	4,491 ms	-37%
DxfReader.Read (first read in process)	8,412 ms	5,936 ms	-29%
DxfReader.ReadEntities	6,698 ms	4,365 ms	-35%
Share of read time in CustomAttribute.AddCustomAttributes (profiler)	53%	0%

The entities read are identical before and after. On net9.0 and net4.8, 2,415 of 2,417 existing tests pass and 2 are skipped.

Tasks done in this PR

• Performance: avoid re-reading DxfCodeValueAttribute via reflection when copying a DxfProperty.
• An evil bug have been defeated.
• Code cleanup and maintenance has been done.

Related Issues / Pull Requests

• None.

Notes for reviewer

• Behaviour should be unchanged:

  • _attributeData and _property are the same instances the old path would have looked up again.
  • Copying the nullable _assignedCode makes AssignedCode resolve exactly as it did on the source (explicit code, single code, or Invalid).
  • The old path's ValueCodes.Contains(code) validation is skipped. The source property already passed it when it was created.
  • StoredValue was not copied before and still isn't.

• Two follow-ups showed up while profiling, intentionally left out of this PR to keep it focused:

  • DxfPropertyBase.SetValue resolves the property type with an IsEquivalentTo chain on every call, which is a large share of the remaining read time. Caching the type category once per property reduced it significantly.
  • DxfClassMap.Create still allocates a full map copy per vertex. The reader never mutates these maps, so it could use the cached instance directly.

  Happy to open separate PRs for either if they're of interest.