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A homography means the same map at any nonzero uniform scale of its entries, but `apply`, `is_invertible` and `inverse` compared w and the determinant with absolute thresholds. A map stored with small entries refused every point and could not be inverted, and a map with large entries drew points past its horizon. - `apply` refuses a point when w is at most 1e-9 times the largest magnitude in the w row. - A map counts as invertible when its determinant is more than 1e-15 times the largest of the six products the determinant sums, taken on a copy rescaled by a power of two. An inverse whose determinant overflows or underflows is taken on that copy.