Cap propagation reach flood fill to 300 km radius

Without a distance limit, the BFS would expand across the entire grid
when conditions were broadly favorable, producing a polygon covering
most of CONUS.

assets/js/propagation_map_hook.js

@@ -113,23 +113,33 @@ function rangeEstimate(score, detail) {
  */
 function propagationReach(gridLookup, startLat, startLon, minScore) {
   const step = 0.125
+  const maxKm = 300
   const snap = (v) => (Math.round(v / step) * step).toFixed(3)
 
   const startKey = `${snap(startLat)},${snap(startLon)}`
   const startScore = gridLookup.get(startKey)
   if (startScore == null || startScore < minScore) return []
 
+  const cosLat = Math.cos(startLat * Math.PI / 180)
+  const kmPerDegLat = 111.0
+  const kmPerDegLon = 111.0 * cosLat
+
   const visited = new Set()
   const reachable = []
   const queue = [startKey]
   visited.add(startKey)
 
-  // BFS flood fill through grid
+  // BFS flood fill through grid, capped at maxKm from origin
   while (queue.length > 0) {
     const key = queue.shift()
     const [latStr, lonStr] = key.split(",")
     const lat = parseFloat(latStr)
     const lon = parseFloat(lonStr)
+
+    const dLat = (lat - startLat) * kmPerDegLat
+    const dLon = (lon - startLon) * kmPerDegLon
+    if (Math.sqrt(dLat * dLat + dLon * dLon) > maxKm) continue
+
     reachable.push({ lat, lon })
 
     // Check 4 neighbors