Fix tube triangulation at line bends

[Copilot]

Line triangulation created duplicate circles with misaligned orientations at bend points, causing gaps/overlaps in the tube geometry.

Changes

LineTriangulator.GetCircles() - Generate one circle per line point instead of two per segment:

• Start/end points: perpendicular to adjacent segment
• Interior points: perpendicular to normalized average of adjacent segment directions (bisector)
• Handle 180° bends where direction vectors cancel

LineTriangulator.GetTriangles() - Remove i++ increment (no longer needed with 1:1 point-to-circle mapping)

Example

// Line with 90° bend: 3 points
var wkt = "LINESTRING(-10 0 0, 0 0 0, 0 10 0)";
var line = (LineString)Geometry.Deserialize<WktSerializer>(wkt);

// Old: 4 circles (2 per segment, 2 overlapping at bend)
// New: 3 circles (1 per point, smooth transition at bend)
var circles = LineTriangulator.GetCircles(line.ToVector3(), radius: 1, segments: 8);

Tests: Added BendTests.cs covering 90°, 135°, 180° bends and multi-bend scenarios.

Original prompt

when triangulating a line something goes wrong in the bends. Make correct tubes in the bends also

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