Kernel rig: fairing leaves a flat patch flat and a face with no free control rows as it was; zero strength is refused

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

tests/kernel-cases/surfaces/KNOWN_FAILING

@@ -3,3 +3,4 @@
 # that fails and is not listed fails it, and so does a listed case that passes: a fix removes its
 # line in the same pull request.
 # Baseline: rig-cases-surfaces 45f69c0e5 with SMLib 60b1c6a, aarch64 Linux.
+cov-fair-single-span-face-has-nothing-free-to-move surface.fair refuses a face of degree 1 one way ("a fairness term does not fit the net's degrees"): kernel/src/fair.rs checks its second-derivative terms against the net's degrees instead of fairing only the direction that has them

tests/kernel-cases/surfaces/cov-fair-flat-patch-stays-flat/expected.json

@@ -0,0 +1,23 @@
+{
+ "sources": [],
+ "what": "fair on a flat boundary patch, with its edges free: a plane has no curvature to even out, so it stays the flat square",
+ "source": "analytic",
+ "method": "the Coons patch of the 20 x 20 square lies in z = 0; fairing moves control points to flatten curvature variation, of which a plane has none, and never past the tolerance: area 400, box z 0..0, 4 free edges",
+ "valid": false,
+ "body_count": 1,
+ "faces": 1,
+ "boundary_edges": 4,
+ "area_mm2": 400.0,
+ "bbox": {
+  "min": [
+   0,
+   0,
+   0
+  ],
+  "max": [
+   20,
+   20,
+   0
+  ]
+ }
+}

tests/kernel-cases/surfaces/cov-fair-flat-patch-stays-flat/recipe.json

@@ -0,0 +1,65 @@
+{
+ "commands": [
+  {
+   "command": "sketch.create",
+   "params": {
+    "plane": {
+     "origin": [
+      0,
+      0,
+      0
+     ],
+     "x_dir": [
+      1,
+      0,
+      0
+     ],
+     "y_dir": [
+      0,
+      1,
+      0
+     ]
+    },
+    "name": "Square"
+   }
+  },
+  {
+   "command": "sketch.rectangle.two_point",
+   "params": {
+    "p0": [
+     0,
+     0
+    ],
+    "p1": [
+     20,
+     20
+    ]
+   }
+  },
+  {
+   "command": "sketch.finish",
+   "params": {}
+  },
+  {
+   "command": "surface.boundary",
+   "params": {
+    "sketch": "Square",
+    "curves": [
+     "l1",
+     "l2",
+     "l3",
+     "l4"
+    ],
+    "body_name": "Patch"
+   }
+  },
+  {
+   "command": "surface.fair",
+   "params": {
+    "body": "Patch",
+    "iterations": 20,
+    "keep_edges": false
+   }
+  }
+ ]
+}

tests/kernel-cases/surfaces/cov-fair-single-span-face-has-nothing-free-to-move/expected.json

@@ -0,0 +1,26 @@
+{
+ "sources": [],
+ "what": "fair on a single-span cubic extrusion holding its edges: every control row is a boundary row or the one next to it, so nothing moves",
+ "source": "analytic",
+ "method": "the face: the Bezier (0,0), (13,8), (27,-8), (40,0) on XZ extruded 20 along -y or +y, B-spline degree (3, 1), one span each way. keep_edges holds the two boundary rows each side and the rows next to them: across the cubic that is all 4 rows, so the surface is unchanged: area = curve length 41.3983 (Gauss-Legendre 64) x 20 = 827.966",
+ "valid": false,
+ "body_count": 1,
+ "faces": 1,
+ "boundary_edges": 4,
+ "area_mm2": 827.9663653036151,
+ "surface_types": [
+  "bspline"
+ ],
+ "splines": [
+  {
+   "degree": [
+    3,
+    1
+   ],
+   "spans": [
+    1,
+    1
+   ]
+  }
+ ]
+}

tests/kernel-cases/surfaces/cov-fair-single-span-face-has-nothing-free-to-move/recipe.json

@@ -0,0 +1,72 @@
+{
+ "commands": [
+  {
+   "command": "sketch.create",
+   "params": {
+    "plane": {
+     "origin": [
+      0,
+      0,
+      0
+     ],
+     "x_dir": [
+      1,
+      0,
+      0
+     ],
+     "y_dir": [
+      0,
+      0,
+      1
+     ]
+    },
+    "name": "Wave"
+   }
+  },
+  {
+   "command": "sketch.spline.control_point",
+   "params": {
+    "points": [
+     [
+      0,
+      0
+     ],
+     [
+      13,
+      8
+     ],
+     [
+      27,
+      -8
+     ],
+     [
+      40,
+      0
+     ]
+    ]
+   }
+  },
+  {
+   "command": "sketch.finish",
+   "params": {}
+  },
+  {
+   "command": "surface.extrude",
+   "params": {
+    "sketch": "Wave",
+    "curves": [
+     "s1"
+    ],
+    "distance": 20,
+    "body_name": "Wave"
+   }
+  },
+  {
+   "command": "surface.fair",
+   "params": {
+    "body": "Wave",
+    "iterations": 10
+   }
+  }
+ ]
+}

tests/kernel-cases/surfaces/cov-fair-zero-strength-refused/expected.json

@@ -0,0 +1,8 @@
+{
+ "sources": [],
+ "what": "fair with strength 0: refused, a step of nothing",
+ "source": "invariants",
+ "method": "strength is the fraction of the fairing step, greater than 0 and at most 1",
+ "outcome": "refused",
+ "refusal": "`strength` must be greater than 0 and at most 1"
+}

tests/kernel-cases/surfaces/cov-fair-zero-strength-refused/recipe.json

@@ -0,0 +1,72 @@
+{
+ "commands": [
+  {
+   "command": "sketch.create",
+   "params": {
+    "plane": {
+     "origin": [
+      0,
+      0,
+      0
+     ],
+     "x_dir": [
+      1,
+      0,
+      0
+     ],
+     "y_dir": [
+      0,
+      0,
+      1
+     ]
+    },
+    "name": "Wave"
+   }
+  },
+  {
+   "command": "sketch.spline.control_point",
+   "params": {
+    "points": [
+     [
+      0,
+      0
+     ],
+     [
+      13,
+      8
+     ],
+     [
+      27,
+      -8
+     ],
+     [
+      40,
+      0
+     ]
+    ]
+   }
+  },
+  {
+   "command": "sketch.finish",
+   "params": {}
+  },
+  {
+   "command": "surface.extrude",
+   "params": {
+    "sketch": "Wave",
+    "curves": [
+     "s1"
+    ],
+    "distance": 20,
+    "body_name": "Wave"
+   }
+  },
+  {
+   "command": "surface.fair",
+   "params": {
+    "body": "Wave",
+    "strength": 0
+   }
+  }
+ ]
+}