fix(traffic): keep a TomTom side from pairing with its own pieces

A one_side line that doubles back within 5 m of itself, such as a
turnaround drawn as one line, matched its own return leg and took a
two-way half share with no partner. Flow index pieces now carry their
source line, and a radiusFor of 0 leaves a piece out of nearest(), so
pairing skips the line's own pieces.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/traffic/roadModes.js

@@ -126,7 +126,8 @@ function pairedSampleShare(segment, index) {
         xs[i - 1] + dx * t,
         ys[i - 1] + dy * t,
         reverse,
-        () => PAIRED_SIDE_RADIUS_M,
+        // A line that doubles back beside itself is not its own pair.
+        (piece) => (piece.flow === segment ? 0 : PAIRED_SIDE_RADIUS_M),
       ).best
     )
       paired++;

src/layers/traffic/roadModes.test.mjs

@@ -163,6 +163,14 @@ test('a TomTom side paired along only part of its length keeps its share', () =>
   assert.equal(weightOfFlow(partner(-97.7434)), 1);
   assert.equal(weightOfFlow(partner(-97.7428)), 0.5);
 });
+test('a TomTom side that doubles back beside itself is not its own pair', () => {
+  // East along `coordinates`, then back west about 1 m north.
+  const hairpin = {
+    ...flow,
+    coords: [...coordinates, [-97.742, 30.26701], [-97.744, 30.26701]],
+  };
+  assert.equal(flowSegmentsToRoads([hairpin])[0].densityWeight ?? 1, 1);
+});
 test('bearing and distance reject nearby crossing and separate OSM roads', () => {
   const parallel = {
     ...main,