fix(perimeters): handle antimeridian queries and incident card anchors

Rebase #1142 onto current main, keeping both sides of the changelog and runtime documentation conflicts. Retain unwrapped containment and circular query coordinates.

Address review feedback on perimeterAnchorDegrees: use circular longitude for the selected outer ring, excluding its closing duplicate. Compare multipart areas along short longitude edges so a small dateline part cannot steal the card anchor from a larger polygon.

Three new card regressions fail before the fix and pass afterward. Coverage includes reversed/rotated rings, vertices at +/-180 and both multipart selection directions. Local validation: 43 focused tests; full suite 7,636 passed, one skipped, zero failures; production build, formatting, module boundaries and whitespace checks pass on Node 24.19.0. Browser tracking validation follows on the QA branch.

src/layers/perimeters/cards.js

@@ -11,20 +11,26 @@ export function containmentAccent(containedPct) {
   return '#8bc34a';
 }
 
-/** Shoelace area (degree², sign dropped) — relative sizes only. */
+/** Shoelace area along short longitude edges (degree², relative sizes only). */
 function ringArea(ring) {
   let sum = 0;
-  for (let i = 0; i < ring.length - 1; i++) {
-    const [x1, y1] = ring[i];
-    const [x2, y2] = ring[i + 1];
+  let [x1, y1] = ring[0];
+  for (let i = 1; i < ring.length; i++) {
+    let delta = ring[i][0] - ring[i - 1][0];
+    if (delta > 180) delta -= 360;
+    else if (delta < -180) delta += 360;
+    const x2 = x1 + delta;
+    const y2 = ring[i][1];
     sum += x1 * y2 - x2 * y1;
+    x1 = x2;
+    y1 = y2;
   }
   return Math.abs(sum / 2);
 }
 
 /**
- * Anchor point for the incident card: the centroid of the largest polygon's
- * outer ring. Degrees, so callers own the Cartesian conversion.
+ * Anchor point for the incident card: the vertex mean of the largest polygon's
+ * outer ring, with circular longitude. Callers own the Cartesian conversion.
  * @param {Array<Array<Array<[number, number]>>>} polygons - Normalized rows' polygons.
  * @returns {{lon: number, lat: number}}
  */
@@ -41,14 +47,17 @@ export function perimeterAnchorDegrees(polygons) {
   }
   // Vertex mean over the closed ring (first == last point dropped): perimeters
   // are compact enough that this stays inside or near the polygon.
-  let lon = 0;
+  let sinLon = 0;
+  let cosLon = 0;
   let lat = 0;
   const count = best.length - 1;
+  const toRad = Math.PI / 180;
   for (let i = 0; i < count; i++) {
-    lon += best[i][0];
+    sinLon += Math.sin(best[i][0] * toRad);
+    cosLon += Math.cos(best[i][0] * toRad);
     lat += best[i][1];
   }
-  return { lon: lon / count, lat: lat / count };
+  return { lon: Math.atan2(sinLon, cosLon) / toRad, lat: lat / count };
 }
 
 /**

src/layers/perimeters/cards.test.mjs

@@ -16,12 +16,79 @@ const tiny = [
   [-107.01, 34.0],
 ];
 
-test('the card anchors at the centroid of the largest polygon', () => {
+test('the card anchors at the vertex mean of the largest polygon', () => {
   const anchor = perimeterAnchorDegrees([[tiny], [square]]);
   assert.ok(Math.abs(anchor.lon - -108.1) < 1e-9);
   assert.ok(Math.abs(anchor.lat - 35.1) < 1e-9);
 });
 
+test('incident cards stay at the dateline for closed, reversed and rotated outlines', () => {
+  const vertices = [
+    [179, 10],
+    [-179, 10],
+    [-179, 12],
+    [179, 12],
+  ];
+  for (let start = 0; start < vertices.length; start++) {
+    const rotated = [...vertices.slice(start), ...vertices.slice(0, start)];
+    for (const points of [rotated, [...rotated].reverse()]) {
+      const anchor = perimeterAnchorDegrees([[points.concat([points[0]])]]);
+      assert.ok(Math.abs(Math.abs(anchor.lon) - 180) < 1e-9);
+      assert.ok(Math.abs(anchor.lat - 11) < 1e-9);
+    }
+  }
+});
+
+test('incident card longitude treats vertices at either sign of 180 as the same meridian', () => {
+  for (const boundary of [-180, 180]) {
+    const ring = [
+      [179, 10],
+      [boundary, 10],
+      [-179, 10],
+      [-179, 12],
+      [179, 12],
+      [179, 10],
+    ];
+    const anchor = perimeterAnchorDegrees([[ring]]);
+    assert.ok(Math.abs(Math.abs(anchor.lon) - 180) < 1e-9);
+    assert.ok(Math.abs(anchor.lat - 10.8) < 1e-9);
+  }
+});
+
+test('multipart cards compare polygon areas along short dateline edges', () => {
+  const ordinary = [
+    [10, 20],
+    [11, 20],
+    [11, 21],
+    [10, 21],
+    [10, 20],
+  ];
+  for (const width of [0.2, 2]) {
+    const crossing = [
+      [180 - width / 2, 10],
+      [-180 + width / 2, 10],
+      [-180 + width / 2, 10 + width],
+      [180 - width / 2, 10 + width],
+      [180 - width / 2, 10],
+    ];
+    for (const ring of [crossing, [...crossing].reverse()]) {
+      for (const polygons of [
+        [[ring], [ordinary]],
+        [[ordinary], [ring]],
+      ]) {
+        const anchor = perimeterAnchorDegrees(polygons);
+        if (width < 1) {
+          assert.ok(Math.abs(anchor.lon - 10.5) < 1e-9);
+          assert.ok(Math.abs(anchor.lat - 20.5) < 1e-9);
+        } else {
+          assert.ok(Math.abs(Math.abs(anchor.lon) - 180) < 1e-9);
+          assert.ok(Math.abs(anchor.lat - 11) < 1e-9);
+        }
+      }
+    }
+  }
+});
+
 test('a full incident row renders name, size, containment, and ages', () => {
   const now = 1758000000000;
   const card = buildIncidentCard(