Complete runnable Vector API (JEP 537) demo

SIMD kernels (square-and-add, masked filter, brightness clamp, FP reduction ordering), an exploratory timing harness, JMH benchmarks with Maven and Maven-free builds, run scripts, README and the captured results with their exact environment.

jmh/run.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+# Build and run the JMH benchmarks (the numbers worth defending).
+#   ./run.sh                -> all benchmarks
+#   ./run.sh MaskedBench    -> one class
+set -euo pipefail
+cd "$(dirname "$0")"
+mvn -q -B clean package
+java --add-modules jdk.incubator.vector -jar target/benchmarks.jar "$@"