fix(AdvancedMarker): resolve race condition with slot content rendering

Fixed timing issue where AdvancedMarker components with custom slot content
would not render correctly on initial load due to template refs being
undefined when the watcher ran with immediate: true.

Changes:
- Add markerRef as watcher dependency to properly track template ref changes
- Add guard clause to wait for markerRef when custom slot content is used
- Add flush: 'post' to ensure DOM updates complete before watcher execution
- Track markerRefChange to trigger updates when template ref becomes available

Fixes rendering issues where:
- AdvancedMarkers appeared as regular Markers until re-renders occurred
- Custom slot content was ignored on initial component mount
- Users needed workarounds like dummy markers to trigger proper rendering

Related to GitHub issue #322.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: HusamElbashir

src/components/AdvancedMarker.vue

@@ -57,13 +57,17 @@ export default defineComponent({
     );
 
     watch(
-      [map, options, pinOptions],
-      async (_, [oldMap, oldOptions, oldPinOptions]) => {
+      [map, options, pinOptions, markerRef],
+      async (_, [oldMap, oldOptions, oldPinOptions, oldMarkerRef]) => {
         const hasOptionChange = !equal(options.value, oldOptions) || !equal(pinOptions.value, oldPinOptions);
-        const hasChanged = hasOptionChange || map.value !== oldMap;
+        const hasMarkerRefChange = markerRef.value !== oldMarkerRef;
+        const hasChanged = hasOptionChange || hasMarkerRefChange || map.value !== oldMap;
 
         if (!map.value || !api.value || !hasChanged) return;
 
+        // If we need custom slot content but markerRef isn't available yet, wait for it
+        if (hasCustomSlotContent.value && !markerRef.value) return;
+
         const { AdvancedMarkerElement, PinElement } = api.value.marker;
 
         if (marker.value) {
@@ -104,6 +108,7 @@ export default defineComponent({
       },
       {
         immediate: true,
+        flush: "post", // Ensure DOM updates happen before this watcher runs
       }
     );