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/types.go

@@ -324,6 +324,21 @@ type Marker struct {
 
 	// RotationSignal drives marker rotation reactively. When set, Rotation is ignored.
 	RotationSignal *via.Signal
+
+	// Offset is a pixel offset from the anchor point as [x, y].
+	Offset [2]float64
+
+	// Scale is a scaling factor for the default marker pin (0 = omit, MapLibre default 1).
+	Scale float64
+
+	// Opacity is the marker opacity 0–1 (0 = omit, MapLibre default 1).
+	Opacity float64
+
+	// OpacityWhenCovered is the marker opacity when behind 3D terrain (0 = omit).
+	OpacityWhenCovered float64
+
+	// ClassName is a CSS class added to the marker container element.
+	ClassName string
 }
 
 // Popup describes a map popup.
@@ -335,6 +350,23 @@ type Popup struct {
 	LngLat          LngLat
 	HideCloseButton bool // true removes the close button (MapLibre shows it by default)
 	MaxWidth        string
+
+	// CloseOnClick controls whether the popup closes on map click.
+	// nil = MapLibre default (true).
+	CloseOnClick *bool
+
+	// CloseOnMove controls whether the popup closes on map move.
+	// nil = MapLibre default (false).
+	CloseOnMove *bool
+
+	// Anchor forces the popup anchor position ("top", "bottom", "left", "right", etc.).
+	Anchor string
+
+	// Offset is a pixel offset from the anchor as [x, y].
+	Offset [2]float64
+
+	// ClassName is a CSS class added to the popup container.
+	ClassName string
 }
 
 // --- Event data ---
@@ -354,14 +386,40 @@ type sourceEntry struct {
 	js string
 }
 
+// MarkerHandle is returned by AddMarker and allows subscribing to marker events.
+type MarkerHandle struct {
+	markerID string
+	m        *Map
+	events   []markerEventEntry
+}
+
+type markerEventEntry struct {
+	event  string
+	signal *via.Signal
+}
+
 type markerEntry struct {
 	id     string
 	marker Marker
+	handle *MarkerHandle
+}
+
+// PopupHandle is returned by ShowPopup and allows subscribing to popup events.
+type PopupHandle struct {
+	popupID string
+	m       *Map
+	events  []popupEventEntry
+}
+
+type popupEventEntry struct {
+	event  string
+	signal *via.Signal
 }
 
 type popupEntry struct {
-	id    string
-	popup Popup
+	id     string
+	popup  Popup
+	handle *PopupHandle
 }
 
 type eventEntry struct {