Test: introduce index_gt_test GTest suite

[yoonseok-kim]

I thought it would be useful to have dedicated unit tests for index_gt, so I added them. I would appreciate your review. :)

Key Changes

• Add index_gt_test.cpp, a GTest-based unit test suite for the low-level index_gt engine
• Wire up the new target via USEARCH_BUILD_TEST_CPP_GT CMake option (GoogleTest v1.17.0 via FetchContent)
• Document build and run instructions in cpp/README.md

Test Coverage

• Parameterized add/search correctness across multiple configs (size, dims, connectivity, ef)
• Search result distance ordering (non-decreasing)
• member_iterator_gt and candidates_iterator_t postfix ++/-- STL semantics
• Prefetch invocation during add and search, including cache-populated metric pattern
• Predicate filtering (even keys, reject-all, exclude-self, combined with prefetch)
• Copy/move semantics, edge cases (1D vectors, minimum connectivity), and stress test (2048 vectors)