Port boolean-crosses from turf (#12)

[trasch]

Summary

Ported turf-boolean-crosses from Turf.js to Swift per OpenGIS Simple Features spec.

Returns true if the intersection of two geometries results in a geometry whose dimension is one less than the maximum dimension of the two source geometries, and the intersection set is interior to both.

Supported combinations

Geometry 1	Geometry 2	Behavior
MultiPoint	LineString	Some points on line interior, some exterior
MultiPoint	Polygon / MultiPolygon	Some points inside, some outside
LineString	LineString	Intersection at non-endpoint
LineString	Polygon / MultiPolygon	Line intersects polygon boundary
Feature	any	Unwraps to geometry automatically
FeatureCollection	any	Checks each contained feature
Unsupported pairs		Returns false

Changes

Sources/GISTools/Algorithms/BooleanCrosses.swift

• Replaced // TODO with full implementation
• GeoJson.crosses(_:) -> Bool — main entry point
• Handles Feature unwrapping and FeatureCollection flattening
• Private helpers for each geometry pair + isPointOnLineSegment utility
• Coordinate3D.isCoincident(to:) helper for endpoint tolerance check

Tests/GISToolsTests/Algorithms/BooleanCrossesTests.swift — 16 tests

• LineString × LineString crossing, non-crossing (parallel), endpoint-touching
• MultiPoint × LineString / MultiPoint × Polygon crossing + non-crossing
• LineString × Polygon / LineString × MultiPolygon crossing + non-crossing
• Feature unwrapping, FeatureCollection support
• Unsupported combinations return false
• Commutativity check

Test results

All 282 tests pass across 60 suites (was 266, +16 new).

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Closes #12