test: reference cases for variable, chord-length and tapered fillets (#776)

Four analytic reference cases for variable, to-zero, chord-length and taper-to-fit fillets, with the formulas in tools/oracle-analytic.

xtask/src/main.rs

@@ -274,7 +274,7 @@ fn cmd_oracle(cli: Option<&str>) -> Result<(), String> {
     }
     let counts: Vec<String> = sources.iter().map(|(k, v)| format!("{v} {k}")).collect();
     let mut md = format!(
-        "# Reference tests\n\nGenerated by `cargo xtask oracle`. Each case in [`tests/oracle/`](../tests/oracle/) is a part described by our own `recipe.json`. SolidCraft replays the recipe through its command engine and compares the result with `expected.json`: body count, volume and area (relative tolerance 1e-3 unless the case states another), centre of mass and bounding box (within 1e-3 of the part's size), face count where the geometry fixes it, and, where the case has a `reference.stl`, the sampled symmetric (Hausdorff) distance between the two surfaces and the principal second moments of both triangulations. Every rebuilt part is also exported as STEP, validated and read back.\n\nWhere the expected values come from (the **Source** column):\n\n- **analytic**: closed-form geometry worked out by hand (prisms, cylinders, cones, spheres, tori and Pappus' theorems, constant-radius fillets on straight edges, sheet-metal bends as annular sectors). The scripts in [`tools/oracle-analytic/`](../tools/oracle-analytic/) compute them and give the formulas.\n- **freecad &lt;version&gt;**: the same recipe built with FreeCAD's public Python scripting API by [`tools/freecad-oracle/`](../tools/freecad-oracle/), an independent kernel run as an external program. FreeCAD is not a dependency of SolidCraft. Where the two kernels legitimately build a different shape (for example the corner patch where fillets meet), only volume and area are compared, within the tolerance the case states.\n\nNo expected value comes from a commercial CAD product.\n\n**{pass} / {} cases pass**; sources: {}. {known} known failure(s) tracked in issues; {missing} case(s) without expected values yet.\n\n| Case | Source | Result | Surface distance mm (max / mean) | Method | Notes |\n|---|---|---|---|---|---|\n",
+        "# Reference tests\n\nGenerated by `cargo xtask oracle`. Each case in [`tests/oracle/`](../tests/oracle/) is a part described by our own `recipe.json`. SolidCraft replays the recipe through its command engine and compares the result with `expected.json`: body count, volume and area (relative tolerance 1e-3 unless the case states another), centre of mass and bounding box (within 1e-3 of the part's size), face count where the geometry fixes it, and, where the case has a `reference.stl`, the sampled symmetric (Hausdorff) distance between the two surfaces and the principal second moments of both triangulations. Every rebuilt part is also exported as STEP, validated and read back.\n\nWhere the expected values come from (the **Source** column):\n\n- **analytic**: closed-form geometry worked out by hand (prisms, cylinders, cones, spheres, tori and Pappus' theorems, fillets on straight edges with a constant radius or one varying along the edge, sheet-metal bends as annular sectors). The scripts in [`tools/oracle-analytic/`](../tools/oracle-analytic/) compute them and give the formulas.\n- **freecad &lt;version&gt;**: the same recipe built with FreeCAD's public Python scripting API by [`tools/freecad-oracle/`](../tools/freecad-oracle/), an independent kernel run as an external program. FreeCAD is not a dependency of SolidCraft. Where the two kernels legitimately build a different shape (for example the corner patch where fillets meet), only volume and area are compared, within the tolerance the case states.\n\nNo expected value comes from a commercial CAD product.\n\n**{pass} / {} cases pass**; sources: {}. {known} known failure(s) tracked in issues; {missing} case(s) without expected values yet.\n\n| Case | Source | Result | Surface distance mm (max / mean) | Method | Notes |\n|---|---|---|---|---|---|\n",
         list.len(),
         counts.join(", ")
     );