Fix PolyBool T-junction stack overflow

[kelo221]

Refs #17

Summary

• Detect weakly-simple PolyBool regions where a vertex lies on a non-adjacent edge.
• Split these singularities into separate simple polygons before hierarchy construction and convex decomposition.
• Prevent point-connected shapes from causing recursive failure in BayazitDecomposer.
• Add Editor regression tests for the singularity split and full convex-generation path.

Validation

• EditMode tests: 2 passed, 0 failed.
• Verified all supplied example projects:
  • Crash.s2d: 4 convex polygons
  • Crash2.s2d: 14 convex polygons
  • ExtrudeFail.s2d: 4 convex polygons
  • PolygonFail.s2d: 4 convex polygons
• No Bayazit stack-overflow warnings or errors occurred.

Closes #17