Fix z-order: zero-pad rect group IDs with D4 to avoid lexicographic sort issue

Agent-Logs-Url: https://github.com/jongalloway/DiagramForge/sessions/13043ef8-b29c-4823-8897-5e634ff57d78

Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

Author: Copilot

src/DiagramForge/Parsers/Mermaid/MermaidSequenceParser.cs

@@ -99,7 +99,7 @@ Node GetOrCreateParticipant(string id)
                 var colorSpec = line["rect ".Length..].Trim();
                 if (TryParseRectColor(colorSpec, out var normalizedColor))
                 {
-                    var groupId = $"sequence:rect:{rectIndex++}";
+                    var groupId = $"sequence:rect:{rectIndex++:D4}";
                     var group = new Group(groupId, string.Empty);
                     group.FillColor = normalizedColor;
                     group.Metadata["sequence:rectGroup"] = true;