feat: support custom HTML/SVG element markers in MapLibre

Add Element, Anchor, and Rotation fields to the Marker struct so users
can render custom icons (ships, circles, buildings) instead of the
default pin. When Element is set, Color is ignored and the raw HTML/SVG
is injected as the marker's DOM element.

Update the example to demonstrate a static ship SVG marker with rotation
and a signal-driven vehicle marker using a custom SVG circle.

maplibre/js.go

@@ -179,10 +179,25 @@ func initScript(m *Map) string {
 // markerBodyJS generates JS to add a marker, assuming `map` is in scope.
 func markerBodyJS(mapID, markerID string, mk Marker) string {
 	var b strings.Builder
+
+	if mk.Element != "" {
+		b.WriteString(fmt.Sprintf(
+			`var _mkEl=document.createElement('div');_mkEl.innerHTML=%s;`,
+			jsonStr(mk.Element)))
+	}
+
 	opts := "{"
-	if mk.Color != "" {
+	if mk.Element != "" {
+		opts += `element:_mkEl.firstElementChild||_mkEl,`
+	} else if mk.Color != "" {
 		opts += fmt.Sprintf(`color:%s,`, jsonStr(mk.Color))
 	}
+	if mk.Anchor != "" {
+		opts += fmt.Sprintf(`anchor:%s,`, jsonStr(mk.Anchor))
+	}
+	if mk.Rotation != 0 {
+		opts += fmt.Sprintf(`rotation:%s,`, formatFloat(mk.Rotation))
+	}
 	if mk.Draggable {
 		opts += `draggable:true,`
 	}