Add jdk27-perf: the same Spring Boot service measured on JDK 25 and JDK 27

Live heap, throughput, p99 and startup across four configurations, including opt-in and opt-out of
compact object headers.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WzVQ6zFh9cuLPj9RSbRH8U

jdk27-perf/scripts/run-one.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Usage: run-one.sh <java-home> <label> [jvm args...]
+# One cold start, then 15 s warmup + 30 s measured load, then a /stats snapshot. One result block.
+set -u
+JH=$1; LABEL=$2; shift 2
+HERE="$(cd "$(dirname "$0")" && pwd)"
+JAR="$HERE/../target/jdk27-perf-1.0.0.jar"
+PORT=${PORT:-18081}
+CLIENT=${JDK_CLIENT:-$JH}
+t0=$(date +%s%N)
+"$JH/bin/java" "$@" -jar "$JAR" --server.port=$PORT --spring.main.banner-mode=off \
+    --logging.level.root=WARN > /dev/null 2>&1 &
+PID=$!
+until [ "$(curl -s -o /dev/null -m 2 -w '%{http_code}' "localhost:$PORT/stats")" = 200 ]; do sleep 0.02; done
+t1=$(date +%s%N)
+echo "== $LABEL"
+echo "startup_ms=$(( (t1 - t0) / 1000000 ))"
+"$CLIENT/bin/java" "$HERE/Load.java" $PORT 15 30 ${THREADS:-4} 2>&1 | grep -v Picked
+curl -s "localhost:$PORT/stats"; echo
+kill $PID 2>/dev/null; wait $PID 2>/dev/null