Port kinks from turf (#48)

[trasch]

Summary

Ported turf-kinks from Turf.js.

GeoJson.kinks() -> FeatureCollection finds all self-intersection points in lines and polygons.

• Supports LineString, MultiLineString, Polygon, MultiPolygon
• Skips adjacent segments and closed-ring start/end (not real kinks)
• Uses existing LineSegment.intersection(_:) for segment intersection
• Returns a FeatureCollection of Point features at each intersection

Tests (7 tests)

Test	Coverage
lineStringNoKinks	Simple line
polygonNoKinks	Simple square
polygonWithKinks	Bow-tie self-intersecting polygon
lineStringWithKinks	Self-intersecting line
multiLineStringWithKinks	Two crossing lines
featureKinks	Feature wrapper
unsupportedGeometryKinks	Point → empty

Test results

273 tests pass across 60 suites (+7 new).

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Closes #48