review comment: remove redundant empty(), use Default instead

JoinQualityMetrics had both a const fn empty() and a derived Default
that produced identical zero-values. Since empty() was not used in a
const context, remove it and use Default::default() at the one callsite
to eliminate the drift risk.

#166 (comment)

Author: altendky

crates/mujou-pipeline/src/mst_join.rs

@@ -1572,7 +1572,7 @@ pub fn join_mst(
     let polylines: Vec<&Polyline> = contours.iter().filter(|c| !c.is_empty()).collect();
 
     if polylines.is_empty() {
-        return (Polyline::new(Vec::new()), JoinQualityMetrics::empty());
+        return (Polyline::new(Vec::new()), JoinQualityMetrics::default());
     }
 
     if polylines.len() == 1 {
@@ -1672,26 +1672,6 @@ pub fn join_mst(
     (polyline, metrics)
 }
 
-impl JoinQualityMetrics {
-    /// Metrics for an empty input (no contours to join).
-    #[must_use]
-    const fn empty() -> Self {
-        Self {
-            mst_edge_count: 0,
-            total_mst_edge_weight: 0.0,
-            max_mst_edge_weight: 0.0,
-            odd_vertices_before_fix: 0,
-            odd_vertices_after_fix: 0,
-            total_retrace_distance: 0.0,
-            total_path_length: 0.0,
-            graph_node_count: 0,
-            graph_edge_count_before_fix: 0,
-            graph_edge_count_after_fix: 0,
-            mst_edge_details: Vec::new(),
-        }
-    }
-}
-
 /// Compute the total Euclidean length of a `Polyline`.
 fn path_polyline_length(polyline: &Polyline) -> f64 {
     let pts = polyline.points();