vt-pinning: diagnosing virtual thread pinning companion code

Reproduces the pinning JEP 491 did not fix (native JNI frames) with a real
native library, alongside the monitor case it did fix. Covers the
jdk.VirtualThreadPinned JFR event, reading pinned vs unmounted virtual
threads from a jcmd Thread.dump_to_file, the measured throughput cost on a
capped carrier pool, and a bounded-executor pattern to contain the blast
radius.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01FhzLY5p6okFva3qsnsRyvM

vt-pinning/output/03-jfr-virtualthreadpinned-event.txt

@@ -0,0 +1,29 @@
+$ java -XX:StartFlightRecording=...,jdk.VirtualThreadPinned#enabled=true,threshold=0ms NativePinningDemo
+
+java.version=25.0.4.1
+jdk.tracePinnedThreads=null
+sleepMillis=200
+done
+
+$ jfr print --events jdk.VirtualThreadPinned --stack-depth 10 target/native-pin.jfr
+
+jdk.VirtualThreadPinned {
+  startTime = 12:02:49.269 (2026-09-30)
+  duration = 200 ms
+  blockingOperation = "LockSupport.park"
+  pinnedReason = "Native or VM frame on stack"
+  carrierThread = "ForkJoinPool-1-worker-1" (javaThreadId = 28)
+  eventThread = "" (javaThreadId = 27, virtual)
+  stackTrace = [
+    java.lang.VirtualThread.parkOnCarrierThread(boolean, long) line: 833
+    java.lang.VirtualThread.parkNanos(long) line: 801
+    java.lang.VirtualThread.sleepNanos(long) line: 983
+    java.lang.Thread.sleepNanos(long) line: 507
+    java.lang.Thread.sleep(long) line: 540
+    com.ankurm.vtpinning.NativePinningDemo.sleepCallback() line: 30
+    com.ankurm.vtpinning.NativePinningDemo.blockingNativeCall()
+    java.lang.VirtualThread.run(Runnable) line: 460
+    jdk.internal.vm.Continuation.enterSpecial(Continuation, boolean, boolean)
+  ]
+}
+