ENH: Store ITK_AUTOLOAD_PATH in ObjectFactoryBase globals - #6985

• The path is initialized from the ITK_AUTOLOAD_PATH environment variable when the ObjectFactoryBase globals are created, and is held there instead of being read from the environment inside LoadDynamicFactories().
• New public static SetAutoloadPath(const std::string &) and GetAutoloadPath(). The setter takes effect if called before the first Initialize() / CreateInstance(), or followed by ReHash().
• ReHash() now uses the stored path rather than re-reading the environment. Code that changes ITK_AUTOLOAD_PATH with putenv after startup and then calls ReHash() must call SetAutoloadPath() instead; itkObjectFactoryTest2 is updated accordingly.
• New standalone test itkObjectFactoryAutoloadPathTest, run two ways: itkObjectFactoryAutoloadPathEnvTest (CTest sets ITK_AUTOLOAD_PATH, nothing configured in main()) and itkObjectFactoryAutoloadPathApiTest (variable unset, SetAutoloadPath() called in main()). Both check that FactoryTestLib is loaded on first use and not loaded after the path is cleared and ReHash() is called.

#6982 asks to replace the compile-time NOT ITK_WRAPPING rule with a control that lives where the hazard is: two ITK builds sharing one process, e.g. an ITK wheel imported inside 3D Slicer whose ITK_AUTOLOAD_PATH names plugins built against Slicer's ITK. This PR does not change when the scan happens. It supplies a runtime control that those options can build on:

• Option 3 (move the control to the host): the host can call SetAutoloadPath("") (or its own plugin directory) before ITK is first used, instead of relying on the process environment. ObjectFactoryBase is already wrapped, so the statics should also be reachable from Python (not yet verified in a wrapped build). In wrapped builds ITK_USE_AUTOLOAD_PATH is off, so the setter only stores the value and no plugins are loaded. It does not require clearing or renaming the environment variable, which other tools in the process may read.
• Option 2 (user-settable control): once the scan is no longer excluded at compile time for wrapped builds, a wrapped build can opt in by setting the path at runtime rather than by a build option.
• Option 1 (refuse mismatched plugins): independent of this PR, but a setter lets an application restrict the search to directories it knows are compatible.

For #6975 (merged), the plugin tests it guards with ITK_USE_AUTOLOAD_PATH rely on ITK_AUTOLOAD_PATH being read by the scan. With a setter, tests no longer need to mutate the process environment to point at a plugin directory.

The new tests are registered inside the ITK_BUILD_SHARED_LIBS AND ITK_USE_AUTOLOAD_PATH block from #6975, so they are skipped in builds where ObjectFactoryBase does not scan the autoload path (e.g. Python-wrapped builds).

Backport: the ENH commit is self-contained and avoids CMake features newer than 3.16.3 (the api test unsets the variable in-process rather than using ENVIRONMENT_MODIFICATION), so it can be cherry-picked to release-5.4; the test guard needs adapting there since ITK_USE_AUTOLOAD_PATH is not on that branch. The ITK 6 migration note is a separate DOC commit that stays on main.

Rebased onto main including #6975. Shared build (BUILD_SHARED_LIBS=ON, ITKCommon only), ctest -R ObjectFactory:

• itkObjectFactoryAutoloadPathEnvTest: passed
• itkObjectFactoryAutoloadPathApiTest: passed
• itkObjectFactoryTest2: passed
• itkObjectFactoryTest3: passed
• itkObjectFactoryOnlyNewTest: passed

Temporary logging confirmed the order in both modes: globals created, then (API mode) SetAutoloadPath(), then the first Initialize() / LoadDynamicFactories() with the configured path. Not run: itkObjectFactoryTest (target not built), static build with the final change, Python-wrapped build, other platforms.

Implemented with an AI coding assistant (Copilot) under the author's direction.