feat: implement Story 2.3 — BPMN diagram type renderer

Add first diagram type renderer with 9 custom BPMN node components,
3 edge types, compound ELK layout for pool/lane hierarchy, BFS path
highlighting on node click, and group container rendering. Includes
review fixes: integrated compound layout into computeLayout pipeline,
wired BFS highlighting via onNodeClick handler, replaced hardcoded
SVG colors with CSS custom properties for dark mode, and added 4
handles to all event nodes for multi-direction layout support.

81 web tests passing (31 new), no regressions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -52,7 +52,7 @@ development_status:
   epic-2: in-progress
   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: backlog
+  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