diff --git a/server/src/atlas/drainage.rs b/server/src/atlas/drainage.rs
index 6e48eebcd..2f7d47c20 100644
--- a/server/src/atlas/drainage.rs
+++ b/server/src/atlas/drainage.rs
@@ -569,7 +569,13 @@ fn build_basins(labels: &[i32], w: usize, h: usize) -> Vec<DrainageBasin> {
 /// `start` (a row-major cell index), clockwise, via Moore-neighbour tracing.
 /// Produces an ordered, 8-connected, non-self-crossing perimeter. The grid edge is
 /// treated as background (no x-wrap) — this is for atlas visualization, not flow.
-fn trace_outer_boundary(labels: &[i32], w: usize, h: usize, basin_id: i32, start: usize) -> Vec<(u16, u16)> {
+fn trace_outer_boundary(
+    labels: &[i32],
+    w: usize,
+    h: usize,
+    basin_id: i32,
+    start: usize,
+) -> Vec<(u16, u16)> {
     // Moore-neighbourhood offsets in clockwise order: N, NE, E, SE, S, SW, W, NW.
     const DIRS: [(i32, i32); 8] = [
         (-1, 0),
@@ -588,7 +594,8 @@ fn trace_outer_boundary(labels: &[i32], w: usize, h: usize, basin_id: i32, start
             && c < w as i32
             && labels[r as usize * w + c as usize] == basin_id
     };
-    let dir_index = |dr: i32, dc: i32| -> usize { DIRS.iter().position(|&o| o == (dr, dc)).unwrap_or(0) };
+    let dir_index =
+        |dr: i32, dc: i32| -> usize { DIRS.iter().position(|&o| o == (dr, dc)).unwrap_or(0) };
 
     let sr = (start / w) as i32;
     let sc = (start % w) as i32;
@@ -660,7 +667,10 @@ mod tests {
             1, 1, 1, 1, //
         ];
         let boundary = trace_outer_boundary(&labels, 4, 4, 1, 0);
-        assert!(boundary.len() >= 8, "expected a real perimeter, got {boundary:?}");
+        assert!(
+            boundary.len() >= 8,
+            "expected a real perimeter, got {boundary:?}"
+        );
         // The defining property the angle-sort violated: consecutive boundary
         // points are 8-adjacent (a genuine contour walk, not crossing chords).
         for w in boundary.windows(2) {