feat: master coordinates moving an instance's players (replace, merge)

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

dMasterServer/MigrationCoordinator.h

@@ -0,0 +1,37 @@
+#ifndef __MIGRATIONCOORDINATOR__H__
+#define __MIGRATIONCOORDINATOR__H__
+
+#include <functional>
+
+#include "InstanceMigration.h"
+#include "RakNetTypes.h"
+
+class Instance;
+
+/**
+ * Master's side of instance migrations (InstanceMigration.h): picks or starts the target instance, holds seats
+ * there, tells the source world to send its players over and passes progress on to whoever asked. The source
+ * instance is "draining" meanwhile, so no new player is sent to it.
+ */
+namespace MigrationCoordinator {
+	// Where statuses go (every world, so the GM who asked hears about it wherever they are); set once at startup
+	void SetReporter(std::function<void(const MigrationStatus&)> reporter);
+
+	// A migration was asked for (a GM command, or later a dashboard); anything but NONE means it was refused (and
+	// reported as FAILED)
+	InstanceMigration::eRefusal Start(const InstanceMigrationRequest& request);
+
+	// A world reported progress on a migration it is running
+	void HandleStatus(const SystemAddress& from, const MigrationStatus& status);
+
+	// A source world sent state to carry over for one player: passed on to the target world as is
+	void HandleCarriedState(const SystemAddress& from, const CarriedPlayerState& state, const unsigned char* data, uint32_t length);
+
+	// A world server went away; migrations it was part of end
+	void OnInstanceGone(const Instance& instance);
+
+	// Every master frame: waits for targets to be ready and times out stuck migrations
+	void Update();
+}
+
+#endif  //!__MIGRATIONCOORDINATOR__H__