feat(v4.3): wall thickness analysis — centroid-offset sampling

- analyze_wall_thickness(): samples surface points, computes min distance
  to vertices/centroids on other faces (no mesh tessellation needed)
- WallThicknessResult: min/max/avg, thin_threshold, thick_threshold
- 3 new tests, 29/29 passing

tests/brep/test_measure.cpp

@@ -224,3 +224,30 @@ TEST(BomTest, EmptyAssembly) {
     auto bom = generate_bom(assy.root);
     EXPECT_TRUE(bom.empty());
 }
+
+// ═══════════════════════════════════════════════════════════
+//  Wall thickness analysis
+// ═══════════════════════════════════════════════════════════
+
+TEST(WallThicknessTest, Box) {
+    auto box = make_box(4, 4, 4);          // 4×4×4 box
+    auto result = analyze_wall_thickness(box, 3);
+    // For a 4×4×4 box, wall thickness ~4.0 (opposite face distance)
+    EXPECT_GT(result.min_thickness, 0.0);
+    EXPECT_GT(result.max_thickness, 0.0);
+    EXPECT_GT(result.avg_thickness, 0.0);
+}
+
+TEST(WallThicknessTest, EmptyBody) {
+    BrepModel empty;
+    auto result = analyze_wall_thickness(empty);
+    EXPECT_NEAR(result.min_thickness, 0.0, 1e-10);
+}
+
+TEST(WallThicknessTest, ThinWallDetection) {
+    // 4×4×0.5 thin plate
+    auto plate = make_box(4, 4, 0.5);
+    auto result = analyze_wall_thickness(plate, 3);
+    EXPECT_GT(result.min_thickness, 0.0);
+    EXPECT_LT(result.min_thickness, 1.0);  // thin
+}