Precompute matched-transition link keys for HTML export

Embed linkKey on matchedTransitions in HTML metadata so explorer highlighting uses the same SVG #link_* suffix as PlantUML, avoiding JS/Java format drift while keeping golden JSON unchanged.

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

state_machine_exporter/src/test/java/click/kamil/springstatemachineexporter/exporter/TransitionLinkKeyTest.java

@@ -0,0 +1,32 @@
+package click.kamil.springstatemachineexporter.exporter;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class TransitionLinkKeyTest {
+
+    @Test
+    void shouldBuildFnFormLinkKeyFromPackageCanonicalIdentifiers() {
+        assertThat(TransitionLinkKey.build(
+                "com.example.order.OrderState.NEW",
+                "com.example.order.OrderEvent.PAY"))
+                .isEqualTo("OrderState_NEW__OrderEvent_PAY");
+    }
+
+    @Test
+    void shouldBuildFqnFormLinkKeyWhenRequested() {
+        assertThat(TransitionLinkKey.build(
+                "com.example.order.OrderState.NEW",
+                "com.example.order.OrderEvent.PAY",
+                EnumFormat.fqn,
+                EnumFormat.fqn))
+                .isEqualTo("com_example_order_OrderState_NEW__com_example_order_OrderEvent_PAY");
+    }
+
+    @Test
+    void shouldBuildEventOnlyLinkKeyWhenSourceMissing() {
+        assertThat(TransitionLinkKey.build(null, "com.example.order.OrderEvent.PAY"))
+                .isEqualTo("__OrderEvent_PAY");
+    }
+}