Port convex hull from turf (#51)

[trasch]

Summary

Ported convex hull algorithm using Andrew's monotone chain algorithm.

Sources/GISTools/Algorithms/ConvexHull.swift

GeoJson.convexHull() -> Polygon? — computes the convex hull of all coordinates in any geometry or feature.

• Uses monotone chain (O(n log n))
• Handles anti-meridian spanning by shifting negative longitudes to [0, 360]
• Deduplicates coordinates via Set
• Returns nil for < 3 distinct points

Tests/ConvexHullTests.swift (7 tests)

Test	Coverage
convexHullSquare	4 corners
convexHullTriangle	3 corners
convexHullWithInteriorPoint	Interior point excluded from hull
convexHullCollinearTop	Collinear points on top edge
convexHullInsufficientPoints	< 3 points → nil
convexHullLineString	From LineString input
convexHullFeature	From Feature wrapper

Test results

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

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Closes #51