Mesh: merge coincident nodes, keeping tags and sets on the survivor

Model::merge_nodes replaces merged nodes by their survivors in elements (node
order kept, so face and edge tags hold) and in set node lists, moves a vertex
tag to an untagged survivor, and refuses a merge that would collapse an
element. fieldcraft_mesh::join::coincident_nodes / merge_coincident find the
nodes within a distance of a lower-labelled survivor, deterministically.
Meshing cell by cell and mirroring use them; node merging commands will too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/model/src/geometry.rs

@@ -244,6 +244,11 @@ impl Classification {
         self.elements.remove(&label);
     }
 
+    /// Tag a node with a tag already interned (the caller checked both).
+    pub(crate) fn tag_node_unchecked(&mut self, label: u32, tag: u32) {
+        self.nodes.insert(label, tag);
+    }
+
     pub(crate) fn forget_node(&mut self, label: u32) {
         self.nodes.remove(&label);
     }