feat(cdclient): make a new CDServer.sqlite off-thread and swap tables in place

CDClientSnapshot converts a copy of the client's fdb into its own
CDServer-<hash>.sqlite with the cdserver migrations applied, on its own
connection. CDClientDatabase::Reconnect opens the new file before letting go
of the old one; CDClientManager::Reload empties every table (old entries kept
alive so references from spawned objects stay valid), retires the old fdb view
and loads again.

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

dDatabase/CDClientDatabase/CDFdb.h

@@ -30,6 +30,12 @@ namespace CDFdb {
 	bool Open(const std::filesystem::path& path, bool allowMapping = true);
 	void Close();
 
+	/**
+	 * Closes the fdb for a CDClient reload, but keeps its view open until the next Retire, so a row or table pointer
+	 * handed out before the reload still reads from memory that is there. Only the latest retired fdb is kept.
+	 */
+	void Retire();
+
 	// The open fdb, or nullptr
 	const FdbReader* Get();