feat(master): watch world zone files and replace instances that loaded old ones

Master hashes each reported file on a worker, polls size and mtime every
world_watch_seconds (a change must hold still for one poll), and replaces
stale instances with the instance migration (Mythran shift, or seamless
with world_reload_seamless=1); empty ones are stopped. Also on WORLD_RELOAD.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dMasterServer/WorldReloader.h

@@ -0,0 +1,47 @@
+#ifndef WORLDRELOADER_H
+#define WORLDRELOADER_H
+
+#include <functional>
+#include <string>
+
+#include "RakNetTypes.h"
+
+class Instance;
+struct MigrationStatus;
+struct WorldFilesReport;
+struct WorldFilesStatus;
+struct WorldReloadRequest;
+
+/**
+ * Master's side of world hot reload (docs/WorldHotReload.md). Worlds report the zone files they loaded (WORLD_FILES);
+ * master watches them (size and mtime every world_watch_seconds, then a hash on a worker thread) and, when one changes
+ * or a GM or the dashboard asks (WORLD_RELOAD), replaces the affected instances with new ones on the files on disk,
+ * moving their players with the instance migration (MigrationCoordinator). Main thread only.
+ */
+namespace WorldReloader {
+	// Where the dashboard's status goes (WORLD_FILES_STATUS)
+	void SetPublisher(std::function<void(const WorldFilesStatus&)> publisher);
+
+	// A world reported its files
+	void HandleReport(const SystemAddress& from, const WorldFilesReport& report);
+
+	// A GM's /reloadworld or the dashboard; who is for the log
+	void HandleRequest(const WorldReloadRequest& request, const std::string& who);
+
+	// Every migration's progress (MigrationCoordinator's observer)
+	void OnMigrationStatus(const MigrationStatus& status);
+
+	// A world server went away
+	void OnInstanceGone(const Instance& instance);
+
+	// The dashboard connected: send it the status again
+	void Republish();
+
+	// Every master frame: polls the files, finishes hashes and reloads what changed
+	void Update();
+
+	// Waits for a running hash (at shutdown)
+	void Shutdown();
+};
+
+#endif // WORLDRELOADER_H