Add jfr-demo: JFR + Mission Control profiling of a live Spring Boot app

Custom OrderProcessed event, CPU/allocation/lock workloads, a diagnostic
endpoint over the JVM's live FlightRecorder state, a JUnit test using the
jdk.jfr.Recording API, and nine captured recordings (flagship, mid-flight
snapshot, isolated vs concurrent threshold comparisons, and a real
dump+stop duplication bug) with their docs/output/*.txt transcripts.

docs/output/01-order-event-test.txt

@@ -0,0 +1,16 @@
+# docs/output/01-order-event-test.txt
+# Generated by OrderProcessedEventTest.capturesOneEventPerOrder()
+# jdk.jfr.Recording started in-process, 20 orders processed, recording stopped and dumped to a temp .jfr file,
+# then read back with jdk.jfr.consumer.RecordingFile.
+
+events recorded:        20
+events with slowReason: 4
+events without reason:  16
+
+first event fields:
+  eventType     = com.ankurm.jfr.OrderProcessed
+  orderId       = TEST-0
+  warehouse     = BLR-2
+  itemCount     = 3
+  duration(ms)  = 5
+  hasStackTrace = true