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/CDClientSnapshot.h

@@ -0,0 +1,48 @@
+#ifndef CDCLIENTSNAPSHOT_H
+#define CDCLIENTSNAPSHOT_H
+
+#include <cstdint>
+#include <filesystem>
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "FdbSnapshot.h"
+
+/**
+ * Makes a new copy of the client's cdclient.fdb and the CDServer.sqlite that goes with it (see FdbSnapshot.h).
+ *
+ * Everything here uses its own files and SQLite connection and never logs, so master runs it on a worker thread and
+ * logs the result on the main thread.
+ */
+namespace CDClientSnapshot {
+	struct Result {
+		bool ok = false;
+		// The copy has the same bytes as the current one; nothing to do
+		bool unchanged = false;
+		std::string error;
+		uint64_t hash = 0;
+		FdbSnapshot::Current current;
+		// What the new fdb changed, one line per table (FdbSnapshot::DescribeChanges)
+		std::vector<std::string> changes;
+		// The cdserver migrations applied to the new CDServer.sqlite
+		std::vector<std::string> migrations;
+	};
+
+	/**
+	 * Converts fdb into a new SQLite file at out and applies every cdserver migration in migrationsDir to it. It is
+	 * written under a temporary name and renamed to out when complete.
+	 */
+	bool MakeSqlite(const std::filesystem::path& fdb, const std::filesystem::path& out, const std::filesystem::path& migrationsDir,
+		std::vector<std::string>& migrations, std::string& error);
+
+	/**
+	 * Copies clientFdb into resServer and, if it differs from currentHash, makes its CDServer-<hash>.sqlite.
+	 *
+	 * @param previousFdb The copy in use now, compared with the new one for Result::changes (may be empty)
+	 */
+	Result Build(const std::filesystem::path& clientFdb, const std::filesystem::path& resServer, const std::filesystem::path& migrationsDir,
+		std::optional<uint64_t> currentHash, const std::filesystem::path& previousFdb);
+};
+
+#endif // CDCLIENTSNAPSHOT_H