Add: radius parameter for distance-bounded search

[andrea-dagostino]

Summary

• The change implements the radius argument for search across the C++ core and Python wrapper, allowing users to exclude results beyond a maximum distance threshold.
• Adds float radius to index_search_config_t, threaded through index_dense_gt::search() and filtered_search() overloads, with post-sort filtering in index_gt::search().
• Wires the existing-but-unused radius parameter in Python Index.search() through _search_in_compiled(), and adds it to Indexes.search() and the module-level search().
• Results with distance > radius are excluded; distance == radius is included. Default is infinity (no filtering, fully backward-compatible).

Changed files

File	What
include/usearch/index.hpp	radius field in index_search_config_t + filtering after sort_ascending()/shrink()
include/usearch/index_dense.hpp	radius param on all 12 search()/filtered_search() overloads + search_()
python/usearch/index.py	radius wired through _search_in_compiled(), Index.search(), Indexes.search(), search()
cpp/test.cpp	test_radius_search() -- default, tight, zero, and infinite radius with L2sq
python/scripts/test_index.py	4 test functions (11 parametrized cases) covering filtering, defaults, edge cases, single/batch

Design notes

• C++ filtering happens after HNSW traversal completes (post-filter on sorted results), not during traversal;
• Python filtering uses np.searchsorted on the ascending-sorted distance arrays in _search_in_compiled(). This handles both the HNSW and brute-force exact search paths without changing lib.cpp;
• lib.cpp (pybind11 bindings) is intentionally not changed -- the module-level search(exact=True) path forwards **kwargs to _exact_search, which doesn't accept radius. Extracting radius as a named parameter in _search_in_compiled() avoids this cleanly.

Tests

• C++ test_radius_search() passes (default, tight, zero, infinite radius)
• C++ full test suite passes (build_debug/test_cpp)
• Python radius tests pass (11/11 cases)
• Python full test suite passes (593 passed, 6 skipped -- skips are pre-existing)