Support structured source/event steps in business flows

Add FlowStep with backward-compatible JSON (strings or objects), precompute linkKey in HTML export, and document precise flow highlighting in flows.json.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -56,11 +56,17 @@ Define sequence of events in `src/main/resources/flows.json`:
   {
     "name": "Order Success",
     "description": "Happy path for order placement",
-    "steps": ["PAY", "CHECK_AVAILABILITY->PENDING", "SHIP"]
+    "steps": [
+      "PAY",
+      "CHECK_AVAILABILITY->PENDING",
+      { "source": "com.example.order.OrderState.PAID", "event": "com.example.order.OrderEvent.SHIP" }
+    ]
   }
 ]
 ```
 
+Event-only strings remain supported but do not highlight in the HTML explorer unless paired with a source state. Use `{source, event}` objects for precise transition highlighting.
+
 ## JSON Structure
 - `metadata.entryPoints`: REST, WebFlux, and JMS entry points.
 - `metadata.callChains`: Trace from API call to machine trigger (`sendEvent`).