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.

internal/examples/maplibre/main.go

@@ -75,14 +75,30 @@ func main() {
 			},
 		})
 
-		// Purple vehicle marker — reads shared Go state
+		// Custom SVG ship marker (static)
+		m.AddMarker("ship", maplibre.Marker{
+			LngLat: maplibre.LngLat{Lng: -122.38, Lat: 37.80},
+			Element: `<svg width="32" height="32" viewBox="0 0 24 24" fill="#1a5276" xmlns="http://www.w3.org/2000/svg">` +
+				`<path d="M3 17h18l-3-8H6L3 17zm9-14l-2 4h4l-2-4zM1 19h22v2H1v-2z"/>` +
+				`</svg>`,
+			Anchor:   "center",
+			Rotation: 45,
+			Popup: &maplibre.Popup{
+				Content: "<strong>Ferry</strong><p>Heading NE</p>",
+			},
+		})
+
+		// Custom SVG vehicle marker — reads shared Go state
 		vehicleLng := c.Signal(-122.43)
 		vehicleLat := c.Signal(37.77)
 
 		m.AddMarker("vehicle", maplibre.Marker{
 			LngSignal: vehicleLng,
 			LatSignal: vehicleLat,
-			Color:     "#9b59b6",
+			Element: `<svg width="20" height="20" viewBox="0 0 20 20" xmlns="http://www.w3.org/2000/svg">` +
+				`<circle cx="10" cy="10" r="9" fill="#9b59b6" stroke="#fff" stroke-width="2"/>` +
+				`</svg>`,
+			Anchor: "center",
 		})
 
 		c.OnInterval(time.Second, func() {