Add tests for 5 nearest/point-on algorithms (#20)

[trasch]

Summary

21 tests across 5 suites for previously untested algorithms.

NearestPointTests (4 tests)

• nearestPointLineString — nearest coordinate to reference
• nearestPointMultiPoint — nearest among multipoint
• nearestPointFeature — nearest point via Feature
• nearestPointEmpty — empty geometry → nil

NearestPointOnFeatureTests (5 tests)

• nearestOnLineString — nearest on line segment surface
• nearestOnPolygonInside — inside → distance 0
• nearestOnPolygonOutside — nearest on boundary
• nearestOnFeature — via Feature wrapper
• nearestOnMultiPoint — nearest point among set

NearestPointToLineTests (3 tests)

• nearestCoordinateOutOf — closest candidate to line
• nearestPointAndDistance — Point variant
• nearestCoordinateOutOfEmpty — empty candidates → nil

PointOnFeatureTests (5 tests)

• pointOnPoint — returns the point itself
• pointOnLineString — centroid on segment
• pointOnPolygon — point on polygon surface
• pointOnMultiPolygon — point on multipolygon surface
• pointOnFeature — via Feature wrapper

PointsWithinPolygonTests (4 tests)

• coordinatesWithin — filter with inside/outside/mixed
• pointsWithin — Point variant
• pointsWithinMultiPolygon — MultiPolygon
• coordinatesWithinEmpty — empty candidates

Test results

287 tests pass across 64 suites (+21 new).

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Closes #20