feat(master): watch the client's cdclient.fdb and reload it on every server

Master copies the fdb at startup, polls its size and mtime every
cdclient_watch_seconds, builds the new copy and CDServer.sqlite on a
worker, switches itself, writes the pointer file, tells every world and
keeps the last two copies.

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

dMasterServer/CDClientReloader.h

@@ -0,0 +1,47 @@
+#ifndef CDCLIENTRELOADER_H
+#define CDCLIENTRELOADER_H
+
+#include <filesystem>
+#include <functional>
+#include <string>
+
+#include "dCommonVars.h"
+
+struct CDClientReload;
+
+/**
+ * Master's side of the CDClient reload (docs/CDClientFdb.md).
+ *
+ * Master keeps a content-addressed copy of the client's cdclient.fdb in resServer and the CDServer.sqlite made from
+ * it, and every server opens those instead of the client's file. It watches the client's file (size and mtime every
+ * cdclient_watch_seconds) and, when it changes or a GM or the dashboard asks, makes a new copy and CDServer.sqlite on a
+ * worker thread, switches itself over and tells every server (CDCLIENT_RELOAD). The last two copies are kept.
+ */
+namespace CDClientReloader {
+	/**
+	 * At startup, once resServer/CDServer.sqlite exists and before CDClientDatabase connects: makes sure the current
+	 * copy of the client's fdb is there (and its CDServer.sqlite, if the fdb changed while master was down) and writes
+	 * the pointer file. Main thread.
+	 *
+	 * @return the CDServer.sqlite to connect to and the fdb copy to map (empty when the client has no loose cdclient.fdb)
+	 */
+	struct Files {
+		std::filesystem::path sqlite;
+		std::filesystem::path fdb;
+	};
+	Files Init(const std::filesystem::path& clientFdb, const std::filesystem::path& resServer, const std::filesystem::path& migrationsDir);
+
+	// Tells every server to switch (CDCLIENT_RELOAD)
+	void SetBroadcast(std::function<void(const CDClientReload&)> broadcast);
+
+	// Checks the client's fdb now; who is for the log
+	void Request(const std::string& who);
+
+	// Every frame: polls the file and finishes a reload whose worker is done. Main thread.
+	void Update();
+
+	// Waits for a running worker (at shutdown)
+	void Shutdown();
+};
+
+#endif // CDCLIENTRELOADER_H