IGES reader checked against Fusion's IGES exports: 75 of 76 reference parts read back exactly; cone tips read whole

`cargo xtask step-corpus --iges` (and `solvecraft-cli step-corpus --iges`) opens every oracle part.igs as File → Open does and compares body count, volume, area and face count with Fusion's measurements, writing docs/iges-import.md. The run found faces closing at an apex (a drill point's cone, a sphere's pole: the seam runs out to the apex and straight back) kept whole and meshed wrongly; they are now split along the iso-line opposite the seam like bands are (18-hole-features and 40-box-sphere-torus pass). The one failure left, 50-sweep-tight-bend, is written by Fusion as five loose, partly degenerate surfaces that Fusion cannot read back as a solid either; documented in the report. Test: a drilled blind hole round-trips through IGES.

Fixes #14

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

crates/kernel/src/iges_in/tests.rs

@@ -141,3 +141,18 @@ fn trimmed_surfaces_sew_into_a_solid() {
     let m = measure(&imp.bodies[0].body).unwrap();
     assert!(rel(m.volume, 6000.0) < 1e-6 && rel(m.area, 2200.0) < 1e-6, "{} {}", m.volume, m.area);
 }
+
+#[test]
+fn a_drill_point_round_trips_through_iges() {
+    // A blind hole with a cone tip: the tip's face closes at its apex (its seam runs out to the
+    // apex and straight back).
+    use solvecraft_geom::{Loop2, Plane, Region2, Vec2};
+    let plate = box_solid(Vec3::new(-20.0, -15.0, 0.0), Vec3::new(20.0, 15.0, 10.0)).unwrap();
+    let profile =
+        Region2 { outer: Loop2::polygon(&[Vec2::new(0.0, 2.0), Vec2::new(4.0, 5.0), Vec2::new(4.0, 11.0), Vec2::new(0.0, 11.0)]), holes: vec![] };
+    let hole = crate::revolve(&Plane::XZ, &[profile], Vec2::ZERO, Vec2::new(0.0, 1.0), std::f64::consts::TAU).unwrap().pop().unwrap();
+    let drilled = boolean(&plate, &hole, BoolOp::Cut).unwrap().unwrap();
+    let v = 12000.0 - std::f64::consts::PI * 16.0 * (5.0 + 1.0);
+    assert!(rel(measure(&drilled).unwrap().volume, v) < 1e-3);
+    round_trip(&drilled);
+}