feat: complete Tier 4 marker/popup options, events, and live drag (#1)

Add missing marker options (offset, scale, opacity, opacityWhenCovered,
className) and popup options (closeOnClick, closeOnMove, anchor, offset,
className).

Return MarkerHandle from AddMarker with OnClick, OnDragStart, OnDrag,
OnDragEnd event methods. Return PopupHandle from ShowPopup with OnOpen,
OnClose event methods.

Upgrade drag signal writeback to fire during drag (throttled via
requestAnimationFrame) in addition to dragend, enabling real-time
position sync across clients.

maplibre/maplibre.go

@@ -135,6 +135,25 @@ func (m *Map) Element(extra ...h.H) h.H {
 				)
 			}
 		}
+		// Hidden inputs for marker event signals
+		if me.handle != nil {
+			for _, ev := range me.handle.events {
+				children = append(children,
+					h.Input(h.Type("hidden"), ev.signal.Bind()),
+				)
+			}
+		}
+	}
+
+	// Hidden inputs for popup event signals
+	for _, pe := range m.popups {
+		if pe.handle != nil {
+			for _, ev := range pe.handle.events {
+				children = append(children,
+					h.Input(h.Type("hidden"), ev.signal.Bind()),
+				)
+			}
+		}
 	}
 
 	children = append(children, extra...)
@@ -290,13 +309,43 @@ func (m *Map) SetLayoutProperty(layerID, name string, value any) {
 // --- Marker methods ---
 
 // AddMarker adds or replaces a marker on the map.
-func (m *Map) AddMarker(id string, marker Marker) {
+// The returned MarkerHandle can be used to subscribe to marker-level events.
+func (m *Map) AddMarker(id string, marker Marker) *MarkerHandle {
+	h := &MarkerHandle{markerID: id, m: m}
 	if !m.rendered {
-		m.markers = append(m.markers, markerEntry{id: id, marker: marker})
-		return
+		m.markers = append(m.markers, markerEntry{id: id, marker: marker, handle: h})
+		return h
 	}
-	js := addMarkerJS(m.id, id, marker)
+	js := addMarkerJS(m.id, id, marker, h.events)
 	m.ctx.ExecScript(js)
+	return h
+}
+
+// OnClick returns a MapEvent that fires when this marker is clicked.
+func (h *MarkerHandle) OnClick() *MapEvent {
+	return h.on("click")
+}
+
+// OnDragStart returns a MapEvent that fires when dragging starts.
+func (h *MarkerHandle) OnDragStart() *MapEvent {
+	return h.on("dragstart")
+}
+
+// OnDrag returns a MapEvent that fires during dragging.
+func (h *MarkerHandle) OnDrag() *MapEvent {
+	return h.on("drag")
+}
+
+// OnDragEnd returns a MapEvent that fires when dragging ends.
+func (h *MarkerHandle) OnDragEnd() *MapEvent {
+	return h.on("dragend")
+}
+
+func (h *MarkerHandle) on(event string) *MapEvent {
+	sig := h.m.ctx.Signal("")
+	ev := &MapEvent{signal: sig}
+	h.events = append(h.events, markerEventEntry{event: event, signal: sig})
+	return ev
 }
 
 // RemoveMarker removes a marker from the map.
@@ -316,13 +365,33 @@ func (m *Map) RemoveMarker(id string) {
 // --- Popup methods ---
 
 // ShowPopup shows a standalone popup on the map.
-func (m *Map) ShowPopup(id string, popup Popup) {
+// The returned PopupHandle can be used to subscribe to popup events.
+func (m *Map) ShowPopup(id string, popup Popup) *PopupHandle {
+	ph := &PopupHandle{popupID: id, m: m}
 	if !m.rendered {
-		m.popups = append(m.popups, popupEntry{id: id, popup: popup})
-		return
+		m.popups = append(m.popups, popupEntry{id: id, popup: popup, handle: ph})
+		return ph
 	}
-	js := showPopupJS(m.id, id, popup)
+	js := showPopupJS(m.id, id, popup, ph.events)
 	m.ctx.ExecScript(js)
+	return ph
+}
+
+// OnOpen returns a MapEvent that fires when the popup opens.
+func (ph *PopupHandle) OnOpen() *MapEvent {
+	return ph.on("open")
+}
+
+// OnClose returns a MapEvent that fires when the popup closes.
+func (ph *PopupHandle) OnClose() *MapEvent {
+	return ph.on("close")
+}
+
+func (ph *PopupHandle) on(event string) *MapEvent {
+	sig := ph.m.ctx.Signal("")
+	ev := &MapEvent{signal: sig}
+	ph.events = append(ph.events, popupEventEntry{event: event, signal: sig})
+	return ev
 }
 
 // ClosePopup closes a standalone popup on the map.