feat(cdclient): servers open master's fdb copy, never the client's file

Worlds, the UGC server and the dashboard open the pair named in
resServer/cdclient-current (CDServer.sqlite without an fdb when there is
none). Worlds switch to a new pair on CDCLIENT_RELOAD between frames.

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

dCommon/FdbSnapshot.h

@@ -62,6 +62,17 @@ namespace FdbSnapshot {
 
 	std::optional<Current> ReadCurrent(const std::filesystem::path& dir);
 
+	/**
+	 * The files a server opens at startup: the pair the pointer file names, when both are there, or CDServer.sqlite
+	 * and no fdb (never the client's own file).
+	 */
+	struct Resolved {
+		std::filesystem::path sqlite;
+		std::filesystem::path fdb; // empty: read CDServer.sqlite only
+	};
+
+	Resolved Resolve(const std::filesystem::path& dir);
+
 	// Writes the pointer file under a temporary name and renames it over the old one (it is never held open)
 	bool WriteCurrent(const std::filesystem::path& dir, const Current& current);