fix(sdf): use GE/LE instead of GT/LT for smooth blend bounds

When d1==d2, smooth blend exactly hits k/4 bound at h=0.5, so
EXPECT_GE/EXPECT_LE are correct instead of EXPECT_GT/EXPECT_LT.

tests/sdf/test_sdf_operations.cpp

@@ -76,7 +76,7 @@ TEST(SdfOperations, SmoothUnionBlending) {
     double k = 0.1;
     double result = op_smooth_union(1.0, 1.0, k);
     EXPECT_LT(result, 1.0);            // blend reduces distance
-    EXPECT_GT(result, 1.0 - k * 0.25); // bound: max reduction is k/4 at h=0.5
+    EXPECT_GE(result, 1.0 - k * 0.25); // bound: max reduction is k/4 at h=0.5
 
     // Far apart — should approach sharp union
     double far_result = op_smooth_union(1.0, 10.0, 0.1);
@@ -92,7 +92,7 @@ TEST(SdfOperations, SmoothIntersectionBlending) {
     double k = 0.1;
     double result = op_smooth_intersection(1.0, 1.0, k);
     EXPECT_GT(result, 1.0);            // blend increases distance
-    EXPECT_LT(result, 1.0 + k * 0.25);
+    EXPECT_LE(result, 1.0 + k * 0.25); // bound: max increase is k/4 at h=0.5
 }
 
 TEST(SdfOperations, SmoothDifferenceFallsBackToSharp) {