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/kernel/src/shell_groups.rs

@@ -79,6 +79,44 @@ fn triangles_meet(a: &[Vec3; 3], b: &[Vec3; 3], gap: f64) -> bool {
     true
 }
 
+/// Whether two closed meshes share material or touch: a boundary triangle of one comes within
+/// `gap` of one of the other's, or one encloses the other.
+pub(super) fn meshes_meet(a: &Mesh, b: &Mesh, gap: f64) -> Result<bool> {
+    if !gap.is_finite() || gap <= 0.0 {
+        return Err(failed("invalid contact clearance"));
+    }
+    let (ba, bb) = (a.bounds(), b.bounds());
+    if ba.is_empty() || bb.is_empty() || boxes_separate(ba, bb, gap) {
+        return Ok(false);
+    }
+    // Only triangles near the other body can touch it.
+    let near = |m: &Mesh, other: Aabb3| -> Result<Vec<([Vec3; 3], Aabb3)>> {
+        let mut out = Vec::new();
+        for t in &m.triangles {
+            let tri = m.tri(t).ok_or_else(|| failed("invalid contact triangle"))?;
+            let bounds = box_of(&tri);
+            if !boxes_separate(bounds, other, gap) {
+                out.push((tri, bounds));
+            }
+        }
+        Ok(out)
+    };
+    let (ta, tb) = (near(a, bb)?, near(b, ba)?);
+    let work = ta.len().checked_mul(tb.len()).ok_or_else(|| failed("contact work exceeds the limit"))?;
+    if work > MAX_PAIR_WORK {
+        return Err(failed("contact work exceeds the limit"));
+    }
+    for (x, xb) in &ta {
+        for (y, yb) in &tb {
+            if !boxes_separate(*xb, *yb, gap) && triangles_meet(x, y, gap) {
+                return Ok(true);
+            }
+        }
+    }
+    // No boundary contact: they meet only if one lies wholly inside the other.
+    Ok(b.positions.first().is_some_and(|p| a.contains(*p)) || a.positions.first().is_some_and(|p| b.contains(*p)))
+}
+
 pub(super) fn groups(meshes: &[Mesh], volumes: &[f64], gap: f64) -> Result<Vec<Vec<usize>>> {
     if meshes.is_empty() || meshes.len() != volumes.len() || meshes.len() > MAX_SHELLS {
         return Err(failed("shell count exceeds the grouping limit or is invalid"));