Reference case for a fillet with a B-spline radius (#810)

Adds oracle case 81 (a box edge rounded at a clamped cubic B-spline radius) with analytic expected values, and passes spline fillets through recipes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/oracle/81-fillet-spline-radius/expected.json

@@ -0,0 +1,25 @@
+{
+  "source": "analytic",
+  "method": "box 40x30x20 with its top front edge (y=0, z=20) rounded at a radius that is a clamped cubic B-spline in u = x/40, knots 0,0,0,0,0.25,0.6,1,1,1,1, control radii 2, 5, 1.5, 4, 1, 3 (r(0) = 2, r(40) = 3). Sections are quarter rounds; V, moments and areas integrated along the edge, exactly per knot span",
+  "body_count": 1,
+  "volume_mm3": 23933.601078437176,
+  "area_mm2": 5150.5132169272065,
+  "center_of_mass": [
+    20.008680496725603,
+    15.039806270911098,
+    9.974065214901927
+  ],
+  "bbox": {
+    "min": [
+      0.0,
+      0.0,
+      0.0
+    ],
+    "max": [
+      40.0,
+      30.0,
+      20.0
+    ]
+  },
+  "faces": 7
+}