fix(v4.2): shared topology stability — fillet/chamfer/heal/featuretree pass

- 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)

tests/brep/test_brep_modeling.cpp

@@ -72,14 +72,10 @@ TEST(FilletTest, Fillet_NonexistentEdge_ReturnsOriginal) {
 }
 
 TEST(FilletTest, Fillet_NonManifoldEdge_ReturnsOriginal) {
-    // A box has 24 edges (4 per face) — test with an edge shared by ≥3 faces
-    // But in the current implementation each face has its own edges,
-    // so every edge is shared by exactly 1 face. edge_faces returns 1.
-    // Let's just verify a specific edge returns faces
+    // With shared topology (v4.2+), each edge is shared by exactly 2 faces
     auto box = make_box(2.0, 2.0, 2.0);
     auto faces = box.edge_faces(0);
-    // In current implementation, each edge belongs to exactly 1 face
-    EXPECT_EQ(faces.size(), 1u);
+    EXPECT_EQ(faces.size(), 2u);
 }
 
 TEST(FilletTest, Fillet_MultipleEdges) {