fix(kernel): a small overlap is not taken for bodies apart or inside (#484)

When sample points say two bodies are apart or one is inside the other, the facetings are now checked for surfaces passing through each other; if they do, the boolean is computed instead of shortcut. If it still can't be intersected, the sampled answer is kept as a flagged, unverified result, and Interference lists such pairs as "could not be checked" with the reason. Fixes #442 and #443.

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

crates/engine/src/cmd/joints_tests.rs

@@ -151,6 +151,57 @@ fn rigid_cycles_solve_and_conflicts_report() {
     assert!(s.execute("joint.create", &json!({"type": "hinge"})).is_err());
 }
 
+/// A 30 x 30 x 3 plate with a through slot whose x = 20 wall carries one rib (0.3 tall, 0.8
+/// wide), and a peg in the slot with 0.15 clearance all round.
+fn ribbed_slot_and_peg(s: &mut Session, peg_x1: f64) {
+    run(s, "sketch.create", json!({"plane": "XY"}));
+    run(s, "sketch.rectangle.two_point", json!({"p0": [0, 0], "p1": [30, 30]}));
+    run(s, "sketch.finish", json!({}));
+    run(s, "solid.extrude", json!({"distance": 3, "body_name": "Plate"}));
+    run(s, "sketch.create", json!({"plane": "XY", "offset": -1}));
+    let slot = json!([[10, 5], [20, 5], [20, 14.6], [19.7, 14.6], [19.7, 15.4], [20, 15.4], [20, 25], [10, 25]]);
+    run(s, "sketch.line", json!({"points": slot, "closed": true}));
+    run(s, "sketch.finish", json!({}));
+    run(s, "solid.extrude", json!({"distance": 5, "operation": "cut", "targets": ["Plate"]}));
+    run(s, "sketch.create", json!({"plane": "XY"}));
+    run(s, "sketch.rectangle.two_point", json!({"p0": [10.15, 5.15], "p1": [peg_x1, 24.85]}));
+    run(s, "sketch.finish", json!({}));
+    run(s, "solid.extrude", json!({"distance": 3, "operation": "new", "body_name": "Peg"}));
+}
+
+/// The peg overlaps only the rib: 0.15 x 0.8 x 3 = 0.36 mm³ (#442).
+#[test]
+fn interference_finds_a_peg_overlapping_a_small_rib() {
+    let mut s = Session::default();
+    ribbed_slot_and_peg(&mut s, 19.85);
+    let i = run(&mut s, "inspect.interference", json!({}));
+    assert_eq!(i["count"], 1, "{i}");
+    assert!((i["interferences"][0]["volume_mm3"].as_f64().unwrap_or(0.0) - 0.36).abs() < 1e-6, "{i}");
+    let (lo, hi) = (&i["interferences"][0]["bbox"]["min"], &i["interferences"][0]["bbox"]["max"]);
+    assert!((lo[0].as_f64().unwrap_or(0.0) - 19.7).abs() < 1e-6 && (hi[0].as_f64().unwrap_or(0.0) - 19.85).abs() < 1e-6, "{i}");
+}
+
+/// An arc revolved up to a post ends on the post's wall, where the two bodies can't be
+/// intersected: the pair is listed as unchecked instead of passing as clear.
+#[test]
+fn interference_lists_a_pair_it_could_not_check() {
+    let mut s = Session::default();
+    run(&mut s, "solid.cylinder", json!({"base": [0, 14, -5], "radius": 4, "height": 15, "body_name": "Post"}));
+    run(&mut s, "sketch.create", json!({"name": "Profile", "plane": "XZ"}));
+    run(&mut s, "sketch.rectangle.two_point", json!({"p0": [10, 0], "p1": [15, 5]}));
+    run(&mut s, "sketch.finish", json!({}));
+    run(&mut s, "solid.revolve", json!({"sketch": "Profile", "axis": {"origin": [0, 0, 0], "dir": [0, 0, 1]}, "to": [4, 14, 2], "body_name": "Arc"}));
+    let i = run(&mut s, "inspect.interference", json!({}));
+    assert_eq!(i["count"], 0, "{i}");
+    let unchecked = i["unchecked"].as_array().cloned().unwrap_or_default();
+    assert_eq!(unchecked.len(), 1, "{i}");
+    assert!(unchecked[0]["reason"].as_str().is_some_and(|r| r.starts_with("not verified") && !r.contains("displacement")), "{i}");
+    // Bodies that are clear of each other, or overlap plainly, leave the list empty.
+    let mut s = two_cubes();
+    let i = run(&mut s, "inspect.interference", json!({}));
+    assert_eq!(i["unchecked"].as_array().map(Vec::len), Some(0), "{i}");
+}
+
 #[test]
 fn motion_links_and_interference() {
     let mut s = two_cubes();