Java 21 baseline across all JVM modules

[Ugbot]

All 13 JVM poms declare Java 17. Moving the baseline to Java 21.

(Java 25 was evaluated and rejected: Flink 2.x supports 17/21, and the pinned Lombok 1.18.38 predates JDK 25 support.)

Scope:

• Bump the version in one place via the shared parent from Build & release plumbing: make the artifacts consumable #1.
• Switch the root pom from <source>/<target> to <release>. With source/target, javac links against the host JDK API, so a jar can compile clean and then NoSuchMethodError on a real baseline JVM. Two ports already do this correctly; the flagship module does not.
• Add maven-enforcer-plugin requireJavaVersion, optionally a toolchain, and commit a Maven wrapper — nothing currently enforces or provisions the JDK.
• Bump Lombok to keep a future JDK 25 evaluation unblocked.
• Add the runtime --add-opens flags to the launcher scripts; they exist for surefire but not for java -jar / mvn exec:java.

Verified: the main module builds and all 770 tests pass on JDK 21 today, so the bump is expected to be mechanical.

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Roadmap epic. Detailed findings, file:line citations and sub-tasks live in the AI-grind tracker (project AGS), which is the source of truth for this work. Local dev — build, test, tracking, profiling — is managed with AI-grind.

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Tracked as AGS-2 in the devtools-mcp tracker.