ci: add build workflow that runs make test (#15)

## 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.

.asf.yaml

diff --git a/.asf.yaml b/.asf.yaml
index cf1630f..175f349 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -48,7 +48,7 @@
     issues: true
     discussions: true
   protected_branches:
-    main: ~
+    main: {}
   pull_requests:
     allow_update_branch: true
     allow_auto_merge: true

.github/workflows/build.yml

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
new file mode 100644
index 0000000..2db4afc
--- /dev/null
+++ b/.github/workflows/build.yml
@@ -0,0 +1,52 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+name: Build
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    name: make test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: '17'
+          cache: maven
+
+      - name: Set up Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Cache cargo build
+        uses: Swatinem/rust-cache@v2
+        with:
+          workspaces: native
+
+      - name: Build native and run JVM tests
+        run: make test