Port pole-of-inaccessibility (polylabel) (#21)

[trasch]

Summary

Ported polylabel (Mapbox) algorithm for finding the pole of inaccessibility.

Sources/GISTools/Algorithms/PoleOfInaccessibility.swift

Polygon.poleOfInaccessibility(precision:) -> Point? — finds the most distant internal point from the polygon outline.

• Uses a priority-queue grid search
• Starts with the polygon centroid and bounding box center as guesses
• Subdivides the most promising cells until precision threshold is met
• Handles holes (all rings are considered for distance)
• Returns nil for polygons without an outer ring

Tests/PoleOfInaccessibilityTests.swift (5 tests)

Test	Coverage
squarePole	Square center
trianglePole	Triangle interior
poleWithPrecision	High and low precision
poleLShaped	Concave L-shape
poleNoOuterRing	Empty polygon → nil

README

Updated pole-of-inaccessibility from TODO to example with test link.

Test results

271 tests pass across 60 suites (+5 new).

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Closes #21