Add Maven build config

[jupblb]

Summary

• add a Maven multi-module reactor alongside the existing Gradle build
• wire Maven packaging for shaded compiler-plugin jars, CLI distribution, snapshots, and Maven plugin descriptor generation
• update CI, Docker, and contributor docs to exercise Maven commands

Verification

• mvn --batch-mode test
• mvn --batch-mode -DskipTests install
• mvn --batch-mode clean -pl scip-java -am -DskipTests package
• mvn --batch-mode clean -pl scip-java -am test
• mvn --batch-mode -pl scip-snapshots -am -DskipTests -PsaveSnapshots verify
• git diff --exit-code scip-snapshots/expected
• examples/maven-example Maven plugin + aggregate smoke test
• nix build .#checks.aarch64-darwin.actionlint -L

Notes

• Draft because Maven Central release/signing remains Gradle-backed for now.