fix: resolve PolyBool T-Junction crash in BayazitDecomposer

[bomlinux92-byte]

Summary

Fix for [BOUNTY 0] PolyBool can cause an editor crash on T-Junctions — Issue #17

The bug: When two shapes look like a B (two loops connected by an infinitely thin point), Boolean operations mistakenly merge them at the singularity, causing a stack overflow crash in BayazitDecomposer.

Fix

Added a preprocessing step in BayazitDecomposer.ConvexPartition that detects self-touching singularities before the recursive Bayazit partitioner runs:

• non-adjacent coincident vertices: detects when a vertex that is not part of an adjacent edge coincides with another vertex
• T-junction style vertex-on-edge touches: detects when a vertex lies exactly on a non-adjacent edge

When a singularity is found, the polygon is split into two valid CCW polygons and each side is partitioned independently. This prevents the recursive Bayazit algorithm from hitting the stack overflow case.

Changes

• Scripts/ShapeEditor/Decomposition/Bayazit/BayazitDecomposer.cs — +168 lines
• Added FindSingularities() method to detect self-touching polygon regions
• Added SplitAtSingularity() method to split polygons at singular vertices/edges
• Added IsSelfTouching() helper to identify problematic polygon configurations

Validation

• git diff --check passes
• Code compiles as a Unity package

Closes #17