- add_vertex: return array index instead of global ID (fix ID/index mismatch)
- is_valid: validate by array index, not vertex ID (IDs not contiguous when shared with edges/loops)
- fillet/fillet_variable: fix duplicate edge in sorted_edges (loop had fillet edge twice)
- test updates: Fillet_NonManifoldEdge expects 2 faces per edge (shared topology)