Maven harness POM fragments

Each fragment is a drop-in <plugin> block. Pin versions; harness.sh warns if they drift out of date by more than one minor.

Properties (top of the POM)

<properties>
    <java.version>25</java.version>
    <maven.compiler.release>25</maven.compiler.release>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>

    <spring-boot.version>4.0.0</spring-boot.version>