Kernel rig: cone cases, a frustum on X and a full cone of negative height with their exact volume, area and centroid, and a zero height refused; a new solids area

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

tests/kernel-cases/solids/KNOWN_FAILING

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+# The solids cases that fail today, one a line: `<case> <why it fails today>`. Sorted. A run with
+# `--known-failing` fails on a case that fails and is not listed here, and on a listed case that
+# passes: a fix removes its line in the same pull request.
+# Baseline: rig-cases-solids-frames 45f69c0e5 with SMLib 60b1c6a, aarch64 Linux.

tests/kernel-cases/solids/PATHS

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+# Source files whose change must rerun the solids area (cones, scale, align, bounding solids).
+apps/solidcraft-cli/src/kernel_cases.rs
+crates/doc/src/eval.rs
+crates/doc/src/eval_more.rs
+crates/engine/src/cmd/features.rs
+crates/engine/src/cmd/features_more.rs
+crates/kernel/src/**
+crates/smlib/**
+tests/kernel-cases/solids/**

tests/kernel-cases/solids/cov-cone-frustum-along-x/expected.json

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+{
+ "sources": [],
+ "what": "a truncated cone (frustum) R10 r5 h20 on the X axis from x=5",
+ "source": "analytic",
+ "method": "frustum: V = pi h/3 (R^2+Rr+r^2) = pi*20*175/3; A = pi(R^2+r^2) + pi(R+r) sqrt(h^2+(R-r)^2); centroid h(R^2+2Rr+3r^2)/(4(R^2+Rr+r^2)) = 7.857 from the base; faces: base, top, side = 3",
+ "body_count": 1,
+ "faces": 3,
+ "volume_mm3": 3665.191429188092,
+ "area_mm2": 1364.1829574548633,
+ "center_of_mass": [
+  12.857142857142858,
+  0,
+  0
+ ],
+ "bbox": {
+  "min": [
+   5,
+   -10,
+   -10
+  ],
+  "max": [
+   25,
+   10,
+   10
+  ]
+ }
+}

tests/kernel-cases/solids/cov-cone-frustum-along-x/recipe.json

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+{
+ "commands": [
+  {
+   "command": "solid.cone",
+   "params": {
+    "radius": 10,
+    "top_radius": 5,
+    "height": 20,
+    "base": [
+     5,
+     0,
+     0
+    ],
+    "axis": "X"
+   }
+  }
+ ]
+}

tests/kernel-cases/solids/cov-cone-full-apex-negative-height/expected.json

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+{
+ "sources": [],
+ "what": "a full cone (top radius 0: an apex) r6, height -12 from a base at z=10: the apex at z=-2",
+ "source": "analytic",
+ "method": "V = pi r^2 h/3 = pi*36*4; A = pi r^2 + pi r sqrt(h^2+r^2); centroid h/4 from the base toward the apex: z = 10-3; faces: base and side = 2",
+ "body_count": 1,
+ "faces": 2,
+ "volume_mm3": 452.3893421169302,
+ "area_mm2": 365.99066584669873,
+ "center_of_mass": [
+  0,
+  0,
+  7
+ ],
+ "bbox": {
+  "min": [
+   -6,
+   -6,
+   -2
+  ],
+  "max": [
+   6,
+   6,
+   10
+  ]
+ }
+}

tests/kernel-cases/solids/cov-cone-full-apex-negative-height/recipe.json

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+{
+ "commands": [
+  {
+   "command": "solid.cone",
+   "params": {
+    "radius": 6,
+    "height": -12,
+    "base": [
+     0,
+     0,
+     10
+    ]
+   }
+  }
+ ]
+}

tests/kernel-cases/solids/cov-cone-zero-height-refused/expected.json

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+{
+ "sources": [],
+ "what": "a cone of height 0 has no volume: refused",
+ "source": "analytic",
+ "method": "the base and the top coincide: no solid, a clean refusal naming the height",
+ "outcome": "refused",
+ "refusal": "height must not be zero"
+}

tests/kernel-cases/solids/cov-cone-zero-height-refused/recipe.json

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+{
+ "commands": [
+  {
+   "command": "solid.cone",
+   "params": {
+    "radius": 5,
+    "height": 0
+   }
+  }
+ ]
+}