feat: add CreateBrushFromPolygonMesh for direct half-edge brush creation

[brainzhangdh]

Summary

Adds ExternalRealtimeCSG.CreateBrushFromPolygonMesh() which builds a RealtimeCSG brush directly from a PolygonMesh by constructing the half-edge connectivity data (ControlMesh) and matching Shape/Surface/TexGen data, bypassing the plane-intersection approach which can fail on certain mesh topologies.

All 6 RealtimeCSGTarget rebuild methods now try the direct PolygonMesh path first and fall back to CreateBrushFromPlanes if validation fails.

What changed

• Added CreateBrushFromPolygonMesh() in ExternalRealtimeCSG.cs:
  • Builds unique vertex list from PolygonMesh
  • Constructs HalfEdge[] with proper twin associations
  • Creates ControlMesh with polygon/vertex/edge indices
  • Builds Shape with per-polygon Surface/TexGen/TexGenFlags
  • Validates via ControlMeshUtility.Validate before creating brush
  • Falls back to plane-based CreateBrushFromPlanes if validation fails
• All 6 rebuild methods (FixedExtrude, ScaledExtrude, RevolveExtrude, LinearStaircase, SplineExtrude, RevolveChopped) now try direct path first

Files changed

• Scripts/Utilities/ExternalRealtimeCSG.cs — new method
• Scripts/Editor/Scene/RealtimeCSGTarget.*.cs — 6 rebuild methods updated

Validation

• Code compiles against Unity 2023.2 (C# 9 / .NET 4.7.2)
• Algorithm follows the half-edge construction pattern from the issue's reference prototype
• Fallback path preserved for degenerate mesh cases

Related issue

Fixes #3.