Add GitHub Actions CI and Dependabot

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: pub
+    directory: /
+    schedule:
+      interval: weekly
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  check:
+    name: Format, analyze, and validate
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: dart-lang/setup-dart@v1
+      - uses: actions/cache@v6
+        with:
+          path: ~/.pub-cache
+          key: pub-${{ runner.os }}-stable-${{ hashFiles('pubspec.yaml') }}
+      - run: dart pub get
+      - run: dart format --output=none --set-exit-if-changed .
+      - run: dart analyze --fatal-infos
+      - run: dart pub publish --dry-run
+
+  test:
+    name: Test (Dart ${{ matrix.sdk }}, ${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        # The floor declared in pubspec.yaml, and the current release.
+        sdk: [3.8.0, stable]
+        include:
+          - os: ubuntu-latest
+            sdk: beta
+    steps:
+      - uses: actions/checkout@v7
+      - uses: dart-lang/setup-dart@v1
+        with:
+          sdk: ${{ matrix.sdk }}
+      - uses: actions/cache@v6
+        with:
+          path: |
+            ~/.pub-cache
+            ~/AppData/Local/Pub/Cache
+          key: pub-${{ runner.os }}-${{ matrix.sdk }}-${{ hashFiles('pubspec.yaml') }}
+      # libzstd_shapes_test.dart compresses its inputs with the zstd CLI and
+      # skips itself when there is none. Only Windows lacks it out of the box.
+      - if: runner.os == 'macOS'
+        run: command -v zstd || brew install zstd
+      - run: dart pub get
+      - run: dart test
+
+  pana:
+    name: pub.dev score
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: dart-lang/setup-dart@v1
+      - run: dart pub global activate pana
+      - run: dart pub global run pana --no-warning --exit-code-threshold 0 .