feat(furnidata): shared lock serializing watcher reindex and editor writes

Emulator/src/main/java/com/eu/habbo/habbohotel/items/FurnidataLock.java

@@ -0,0 +1,13 @@
+package com.eu.habbo.habbohotel.items;
+
+import java.util.concurrent.locks.ReentrantLock;
+
+/**
+ * One process-wide lock serializing every furnidata reindex and every editor-driven
+ * furnidata write, so an editor write never races the file watcher's reindex and the
+ * volatile index is never observed mid-swap by two writers.
+ */
+public final class FurnidataLock {
+    public static final ReentrantLock LOCK = new ReentrantLock();
+    private FurnidataLock() {}
+}

Emulator/src/main/java/com/eu/habbo/habbohotel/items/FurnidataWatcher.java

@@ -116,6 +116,8 @@ public class FurnidataWatcher {
     }
 
     private void onChange() {
+        FurnidataLock.LOCK.lock();
+        try {
             Path source = this.provider.getSource();
             if (source == null) return;
 
@@ -136,6 +138,9 @@ public class FurnidataWatcher {
             broadcast(composer);
             LOGGER.info("FurnidataWatcher: broadcast {} ({} entries)",
                 delta.size() > this.deltaCap ? "reload-hint" : "delta", delta.size());
+        } finally {
+            FurnidataLock.LOCK.unlock();
+        }
     }
 
     private void broadcast(FurnitureDataReloadComposer composer) {