mesh.cells: sweep the cells of a body together, conforming with no seeds (#288)

hex_mesh_swept_cells lays out one cross-section for several cells: every
cell's walls (its cut faces included) are imprinted together, every cell's
levels are levels of all, one interval assignment and one set of layers,
and each region of each level interval is filled by the cell it lies in.
Cells meet node for node across every cut, T-junctions included.

mesh.cells sweeps the cells of each design body together first and meshes
cell by cell only what does not sweep together (saying why); the reply
gives every cell's volume, scaled Jacobian and edge ratio.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/geometry/src/design.rs

@@ -76,6 +76,16 @@ impl Design {
     pub fn body(&self, name: &str) -> Option<&Shape> {
         self.bodies.iter().find(|b| b.name == name)
     }
+
+    /// The body of the design as opened that `name` is, or that partitions cut it from
+    /// (`Base` for `Base/a/b1`).
+    pub fn source_of(&self, name: &str) -> Option<&str> {
+        self.source
+            .iter()
+            .map(|b| b.name.as_str())
+            .filter(|s| name == *s || name.strip_prefix(s).is_some_and(|rest| rest.starts_with('/')))
+            .max_by_key(|s| s.len())
+    }
 }
 
 /// Open a design from its file bytes (a `.solidcraft` container or a JSON design) and rebuild it.