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/scripts/build-native.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Compiles src/main/c/vtpinning.c into libvtpinning.so, linked against the JNI headers of
+# whichever JDK is on JAVA_HOME (any JDK's jni.h works - the JNI ABI is stable across versions;
+# this repo built it once against JDK 21's headers and loads it fine under JDK 21 and JDK 25).
+# The .so is NOT committed to the repo; run this before the other scripts.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+: "${JAVA_HOME:?Set JAVA_HOME to a JDK install (its include/ dir must have jni.h)}"
+
+mkdir -p target/native
+gcc -shared -fPIC \
+    -I"$JAVA_HOME/include" \
+    -I"$JAVA_HOME/include/linux" \
+    -o target/native/libvtpinning.so \
+    src/main/c/vtpinning.c
+
+echo "Built target/native/libvtpinning.so"