refactor: split integration tests by feature

[mrhooray]

Summary

• split the monolithic tests/kdtree.rs integration suite into tests/feature_nearest.rs and tests/feature_within.rs, keeping the original scenarios intact
• add tests/_util.rs for shared fixtures/assertion helpers and rename supporting suites (distance_call_count.rs, serde.rs) so the test tree stays organized
• run cargo test to confirm coverage matches the previous layout

Testing

• cargo test