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.cpp

@@ -7,6 +7,7 @@
 
 namespace {
 	std::unique_ptr<FdbReader> g_Fdb;
+	std::unique_ptr<FdbReader> g_Retired;
 
 	// Column name -> index for each table handed out by GetTable, so RowFields looks up names in O(1)
 	std::unordered_map<const FdbReader::Table*, std::unordered_map<std::string, int32_t>> g_ColumnIndices;
@@ -127,6 +128,11 @@ void CDFdb::Close() {
 	g_Fdb.reset();
 }
 
+void CDFdb::Retire() {
+	g_ColumnIndices.clear();
+	if (g_Fdb) g_Retired = std::move(g_Fdb);
+}
+
 const FdbReader* CDFdb::Get() {
 	return g_Fdb.get();
 }