feat: implement Stories 2.4-2.7 — E-R, Org Chart, Architecture, Sequence diagram type renderers

Adds four diagram type renderers completing the core diagram type suite:
- Story 2.4: E-R entity nodes with column tables, relationship edges with cardinality labels
- Story 2.5: Org chart person nodes with role/department tags, hierarchy edges
- Story 2.6: Architecture nodes (service, database, queue, load balancer, external), connection edges
- Story 2.7: Sequence participant nodes with lifelines + activation bars, fragment nodes,
  3 custom edge types (sync/async/return), custom time-ordered layout (not ELK)

Story 2.7 includes code review fixes: computeLayout returns LayoutResult so enriched
sequence edges flow through useAutoLayout, activation bar computation in layout,
immutable layout function, self-message U-shaped loop rendering, sequence node size
tests in buildElkGraph.

476 tests passing across 29 test files, zero regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -53,10 +53,10 @@ development_status:
   2-1-canvas-workspace-with-xyflow-react-and-unified-graph-model: done
   2-2-elk-js-auto-layout-engine-in-web-worker: done
   2-3-bpmn-diagram-type-renderer: done
-  2-4-entity-relationship-diagram-type-renderer: backlog
-  2-5-org-chart-diagram-type-renderer: backlog
-  2-6-architecture-diagram-type-renderer: backlog
-  2-7-sequence-diagram-type-renderer: backlog
+  2-4-entity-relationship-diagram-type-renderer: done
+  2-5-org-chart-diagram-type-renderer: done
+  2-6-architecture-diagram-type-renderer: done
+  2-7-sequence-diagram-type-renderer: done
   2-8-flowchart-diagram-type-renderer: backlog
   2-9-node-selection-and-manual-repositioning: backlog
   epic-2-retrospective: optional