Summary

• Adds .github/workflows/build.yml running on pushes to main and on PRs targeting main.
• Sets up Temurin JDK 17 and a stable Rust toolchain, caches Maven (~/.m2) and cargo (native/target) artifacts, then runs make test.

Rationale

The project currently has no CI: format.yml was removed in #7 to clear a deadlock when GitHub Actions wasn't yet enabled, and #11 nulled out the stale branch protection rules left behind. With Actions now in place, we need at least one workflow that exercises the build on every PR so regressions surface before merge.

make test is the canonical full build entry point documented in README.md and CONTRIBUTING.md — it depends on the native target (so it builds the Rust crate first) and then runs the JVM JUnit suite, which is exactly what CI needs to cover.

What's in this PR

• .github/workflows/build.yml: single build job on ubuntu-latest, JDK 17 (Temurin) with Maven cache via actions/setup-java, Rust stable via dtolnay/rust-toolchain, cargo cache via Swatinem/rust-cache scoped to the native workspace, then make test.

Not in this PR

• Spotless / Apache RAT / clippy / cargo fmt checks. These run at Maven's verify phase or via separate cargo commands and aren't wired into the Makefile yet; they can be added in a follow-up once we decide whether to extend the Makefile or invoke them directly from CI.
• Matrix builds across OS or JDK versions — start minimal, expand if needed.