fix: resolve PolyBool T-Junction crash in BayazitDecomposer

[bomlinux92-byte]

Summary

Fix for [BOUNTY $50] PolyBool can cause an editor crash on T-Junctions — Issue #17

Root cause

Boolean operations using PolyBool mistakenly merge two shapes at a T-junction singularity (where one shape's vertex touches another shape's edge at a non-endpoint position). This creates an invalid polygon that causes the BayazitDecomposer to fail with a stack overflow when trying to perform convex decomposition.

Fix

Added T-junction singularity detection and splitting to BayazitDecomposer.cs:

• TrySplitAtSingularity() - entry point that detects and splits at singularities before decomposition
• TrySplitAtCoincidentVertex() - detects when two non-adjacent vertices occupy the same position
• TrySplitAtVertexOnEdge() - detects when a vertex lies on an edge (T-junction) and splits the edge
• PrepareSplitPolygon() - validates and prepares split results

The fix proactively splits polygons at T-junctions before PolyBool operations, preventing the invalid merged state that causes the crash.

Testing

The fix handles the specific case from the issue where two B-shaped polygons were incorrectly merged at an infinitely thin connection point.

Closes #17