perf: reduce choropleth re-renders with 50% padding + bounds check

Only re-renders when viewport moves >30% outside pre-rendered area.
50% padding means most small pans stay within the existing image.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/map-embed.html

@@ -109,9 +109,9 @@ body { margin: 0; overflow: hidden; }
       var east = Math.min(180, bounds.getEast());
       var south = Math.max(-MAX_LAT, bounds.getSouth());
       var north = Math.min(MAX_LAT, bounds.getNorth());
-      // Pad bounds slightly
+      // Large padding so small pans don't trigger re-render
-      var lonPad = (east - west) * 0.1;
+      var lonPad = (east - west) * 0.5;
-      var latPad = (north - south) * 0.1;
+      var latPad = (north - south) * 0.5;
       west = Math.max(-180, west - lonPad);
       east = Math.min(180, east + lonPad);
       south = Math.max(-MAX_LAT, south - latPad);
@@ -148,6 +148,7 @@ body { margin: 0; overflow: hidden; }
 
       var url = offscreen.toDataURL('image/png');
       var coords = [[west, north], [east, north], [east, south], [west, south]];
+      lastBounds = [west, north, east, south];
 
       if (_ifkMap.getSource('choropleth-img')) {
         _ifkMap.getSource('choropleth-img').updateImage({ url: url, coordinates: coords });
@@ -165,8 +166,20 @@ body { margin: 0; overflow: hidden; }
       }
     }
 
+    var lastBounds = null;
+    function needsRerender() {
+      if (!lastBounds) return true;
+      var b = _ifkMap.getBounds();
+      // Only re-render if viewport moved >30% outside last rendered area
+      var threshold = 0.3;
+      var lw = lastBounds[2] - lastBounds[0], lh = lastBounds[1] - lastBounds[3];
+      return b.getWest() < lastBounds[0] + lw * threshold ||
+             b.getEast() > lastBounds[2] - lw * threshold ||
+             b.getNorth() > lastBounds[1] - lh * threshold ||
+             b.getSouth() < lastBounds[3] + lh * threshold;
+    }
     renderChoropleth();
-    _ifkMap.on('moveend', renderChoropleth);
+    _ifkMap.on('moveend', function() { if (needsRerender()) renderChoropleth(); });
 
     // Keep GeoJSON source for hover/click interaction (invisible)
     _ifkMap.addSource('countries', { type: 'geojson', data: geo });