fix: render choropleth as single image overlay — eliminates all tile seams

GeoJSON sources are internally tiled by geojson-vt, causing seams at
tile boundaries during panning. Solution: pre-render the choropleth
to a canvas, convert to PNG data URL, and add as a MapLibre image
source. Single image = zero tiles = zero seams.

GeoJSON source kept invisible for hover/click hit testing.

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

static/map-embed.html

@@ -24,7 +24,8 @@ body { margin: 0; overflow: hidden; }
   var LANG = 'en';
   try { var m = parent.location.pathname.match(/^\/(de|fr|en)\//); if (m) LANG = m[1]; } catch(e) {}
 
-  var STATUS_COLORS = { enforced: '#667eea', passed: '#764ba2', progress: '#a5b4fc', guidelines: '#e0e7ff' };
+  // Pre-blended with base map — fully opaque to prevent tile seam artifacts from transparency
+  var STATUS_COLORS = { enforced: '#9cabf1', passed: '#a68ac3', progress: '#c5cefd', guidelines: '#ebefff' };
   var STATUS_LABELS = {
     en: { enforced: 'Enforced', passed: 'Passed', progress: 'In Progress', guidelines: 'Guidelines' },
     de: { enforced: 'In Kraft', passed: 'Verabschiedet', progress: 'In Bearbeitung', guidelines: 'Richtlinien' },
@@ -41,6 +42,7 @@ body { margin: 0; overflow: hidden; }
     center: [20, 20],
     zoom: 2,
     scrollZoom: false,
+    fadeDuration: 0,
     style: 'https://maps.clicksports.de/styles/klokantech-basic/style.json'
   });
   _ifkMap.addControl(new maplibregl.NavigationControl({ showCompass: false }), 'top-right');
@@ -65,28 +67,57 @@ body { margin: 0; overflow: hidden; }
     geo.features.forEach(function(f) {
       var id = String(f.id).padStart(3, '0');
       var c = byIsoNum[id];
-      f.properties.fillColor = c ? STATUS_COLORS[c.status] : '#dce4ec';
-      f.properties.fillOpacity = c ? 0.65 : 0.1;
+      f.properties.fillColor = c ? STATUS_COLORS[c.status] : 'transparent';
+      f.properties.fillOpacity = c ? 1 : 0;
       f.properties.isoNum = id;
     });
 
-    _ifkMap.addSource('countries', { type: 'geojson', data: geo });
+    // Render choropleth to a canvas, then add as single image source (NOT tiled)
+    var W = 2048, H = 1024;
+    var offscreen = document.createElement('canvas');
+    offscreen.width = W; offscreen.height = H;
+    var ctx = offscreen.getContext('2d');
 
-    _ifkMap.addLayer({
-      id: 'choropleth-fill', type: 'fill', source: 'countries',
-      paint: {
-        'fill-color': ['get', 'fillColor'],
-        'fill-opacity': ['get', 'fillOpacity'],
-        'fill-antialias': false
-      }
+    // Draw country fills using Equirectangular projection
+    geo.features.forEach(function(f) {
+      var id = String(f.id).padStart(3, '0');
+      var c = byIsoNum[id];
+      if (!c) return;
+      ctx.fillStyle = STATUS_COLORS[c.status];
+      ctx.beginPath();
+      var coords = f.geometry.type === 'Polygon' ? [f.geometry.coordinates] : f.geometry.coordinates;
+      coords.forEach(function(poly) {
+        poly.forEach(function(ring) {
+          for (var i = 0; i < ring.length; i++) {
+            var x = (ring[i][0] + 180) / 360 * W;
+            var y = (90 - ring[i][1]) / 180 * H;
+            if (i === 0) ctx.moveTo(x, y);
+            else ctx.lineTo(x, y);
+          }
+          ctx.closePath();
+        });
+      });
+      ctx.fill();
+    });
+
+    // Add as image source — single image, NO tiling
+    _ifkMap.addSource('choropleth-img', {
+      type: 'image',
+      url: offscreen.toDataURL('image/png'),
+      coordinates: [[-180, 85.05], [180, 85.05], [180, -85.05], [-180, -85.05]]
     });
     _ifkMap.addLayer({
-      id: 'choropleth-border', type: 'line', source: 'countries',
-      paint: { 'line-color': '#94a3b8', 'line-width': 0.5 }
+      id: 'choropleth-fill',
+      type: 'raster',
+      source: 'choropleth-img',
+      paint: { 'raster-opacity': 0.85, 'raster-fade-duration': 0 }
     });
+
+    // Keep GeoJSON source for hover/click interaction (invisible)
+    _ifkMap.addSource('countries', { type: 'geojson', data: geo });
     _ifkMap.addLayer({
       id: 'choropleth-hover', type: 'fill', source: 'countries',
-      paint: { 'fill-color': '#667eea', 'fill-opacity': 0.2 },
+      paint: { 'fill-color': '#667eea', 'fill-opacity': 0.2, 'fill-antialias': false },
       filter: ['==', 'isoNum', '']
     });