Fix GetArbitraryAxis threshold for normals near +Z

[jarrett-meyer]

Bug

Matrix4.GetArbitraryAxis(XYZ zaxis) decides which world axis to cross with the normal using:

if (Math.Abs(zaxis.X) < (1 / 64) && Math.Abs(zaxis.Y) < (1 / 64))

1 / 64 is integer division, so the threshold is always 0 and the condition can never be true. For any normal whose X and Y components are both below 1/64 (but which is not exactly +Z or -Z), the method computes the X axis from Wz x N instead of Wy x N. This does not follow the DXF arbitrary axis algorithm, so entities with a normal close to +Z get an OCS rotated by about 90 degrees.

Matrix3.ArbitraryAxis already uses 1 / 64.0 and is not affected.

How to reproduce

XYZ n = new XYZ(0.01, 0, 1).Normalize();
Matrix4 m = Matrix4.GetArbitraryAxis(n);
// Expected X axis: Cross(AxisY, n) ~ (1, 0, -0.01)
// Actual X axis:   Cross(AxisZ, n) = (0, 1, 0)

Fix

Use 1.0 / 64 so the comparison is done in floating point.

Tests

Added CSMath.Tests/Matrix4Tests.cs with two cases: a normal near +Z (uses Wy x N; fails before this change) and a general normal (uses Wz x N). The full CSMath.Tests suite passes (292 tests).

🤖 Generated with Claude Code