fix: join merges every body the tool meets

A Join unioned its tools into the first target only, leaving other target
bodies the tool overlapped or touched as separate, interpenetrating bodies.
Each tool now merges with every target it meets (shared material or face
contact, tested on the operands' boundaries) into one body under the first
one's name; absorbed bodies are removed and their threads follow. A tool that
meets no target still joins the first target as a separate lump. Sheet metal
and mesh bodies are never absorbed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J53sR6gEaVt56soFDpTck8

crates/engine/src/tests.rs

@@ -47,6 +47,50 @@ fn join_extrude_from_a_chamfered_top_face() {
     assert!(rel(volume(&mut s), base + PI * 64.0 * 5.0) < 5e-4);
 }
 
+/// A join that bridges two bodies makes them one body, not an overlap of the first and the second.
+#[test]
+fn join_bridging_two_bodies_merges_them() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10}));
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10, "corner": [20, 0, 0], "operation": "new_body"}));
+    run(&mut s, "solid.box", json!({"length": 20, "width": 4, "height": 4, "corner": [5, 3, 3], "operation": "join"}));
+    let st = s.model.state();
+    assert_eq!(st.bodies.len(), 1, "both boxes and the bar are one body");
+    assert_eq!(st.bodies[0].name, "Body1");
+    assert_eq!(st.bodies[0].body.lumps().unwrap().len(), 1);
+    // 2 boxes + the bar's 10 mm outside them.
+    assert!(rel(volume(&mut s), 2000.0 + 10.0 * 16.0) < 1e-6);
+}
+
+/// A join touching only the second body merges into that body and leaves the first alone.
+#[test]
+fn join_merges_into_the_body_it_meets() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10}));
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10, "corner": [20, 0, 0], "operation": "new_body"}));
+    // Sits on Body2's top face: a face contact, no shared volume.
+    run(&mut s, "solid.box", json!({"length": 4, "width": 4, "height": 4, "corner": [23, 3, 10], "operation": "join"}));
+    let st = s.model.state();
+    assert_eq!(st.bodies.len(), 2);
+    let b1 = st.bodies.iter().find(|b| b.name == "Body1").unwrap();
+    let b2 = st.bodies.iter().find(|b| b.name == "Body2").unwrap();
+    assert_eq!(b1.body.face_count(), 6, "Body1 is untouched");
+    assert_eq!(b2.body.lumps().unwrap().len(), 1);
+    assert!(rel(volume(&mut s), 2064.0) < 1e-6);
+}
+
+/// A join that meets no body still joins the first body, as a separate lump.
+#[test]
+fn join_meeting_nothing_keeps_the_first_body() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10}));
+    run(&mut s, "solid.box", json!({"length": 4, "width": 4, "height": 4, "corner": [40, 0, 0], "operation": "join"}));
+    let st = s.model.state();
+    assert_eq!(st.bodies.len(), 1);
+    assert_eq!(st.bodies[0].body.lumps().unwrap().len(), 2);
+    assert!(rel(volume(&mut s), 1064.0) < 1e-6);
+}
+
 mod component_frames;
 mod mirror_planes;
 mod primitive_points;