chore: import upstream snapshot with attribution

.github/workflows/diff_shades.yml

@@ -0,0 +1,240 @@
+name: diff-shades
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - src/**
+      - pyproject.toml
+      - scripts/diff_shades_gha_helper.py
+      - .github/workflows/diff_shades.yml
+      - .github/workflows/diff_shades_comment.yml
+
+  pull_request:
+    paths:
+      - src/**
+      - pyproject.toml
+      - scripts/diff_shades_gha_helper.py
+      - .github/workflows/diff_shades.yml
+      - .github/workflows/diff_shades_comment.yml
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_id }}
+  cancel-in-progress: true
+
+env:
+  HATCH_BUILD_HOOKS_ENABLE: "1"
+  # Clang is less picky with the C code it's given than gcc (and may generate faster
+  # binaries too).
+  CC: clang-18
+
+permissions: {}
+
+jobs:
+  configure:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-config.outputs.matrix }}
+
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
+
+      - name: Set up Python
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6.3.0
+        with:
+          python-version: "3.14"
+          pip-version: "25.3"
+          pip-install: --group diff-shades --group diff-shades-comment
+
+      - name: Calculate run configuration & metadata
+        id: set-config
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+        run: python scripts/diff_shades_gha_helper.py config
+
+  analysis-base:
+    name: analysis / base / ${{ matrix.mode }}
+    needs: configure
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include: ${{ fromJson(needs.configure.outputs.matrix) }}
+
+    steps:
+      - name: Checkout this repository (full clone)
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          # The baseline revision could be rather old so a full clone is ideal.
+          fetch-depth: 0
+          persist-credentials: false
+
+      - name: Set up Python
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6.3.0
+        with:
+          python-version: "3.14"
+          pip-version: "25.3"
+          pip-install: --group diff-shades
+
+      - name: Configure git
+        run: |
+          git config user.name "diff-shades-gha"
+          git config user.email "diff-shades-gha@example.com"
+
+      - name: Attempt to use cached baseline analysis
+        id: baseline-cache
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ matrix.baseline-analysis }}
+          key: ${{ matrix.baseline-cache-key }}
+
+      - name: Build and install baseline revision
+        if: steps.baseline-cache.outputs.cache-hit != 'true'
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+        run: |
+          ${{ matrix.baseline-setup-cmd }}
+          python -m pip install .
+
+      - name: Analyze baseline revision
+        if: steps.baseline-cache.outputs.cache-hit != 'true'
+        run:
+          diff-shades analyze ${{ matrix.baseline-analysis }} --work-dir projects-cache/
+          --force-${{ matrix.style }}-style -v
+
+      - name: Upload baseline analysis
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: ${{ matrix.baseline-analysis }}
+          path: ${{ matrix.baseline-analysis }}
+
+  analysis-target:
+    name: analysis / target / ${{ matrix.mode }}
+    needs: configure
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include: ${{ fromJson(needs.configure.outputs.matrix) }}
+
+    steps:
+      - name: Checkout this repository (full clone)
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          # The baseline revision could be rather old so a full clone is ideal.
+          fetch-depth: 0
+          persist-credentials: false
+
+      - name: Set up Python
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6.3.0
+        with:
+          python-version: "3.14"
+          pip-version: "25.3"
+          pip-install: --group diff-shades
+
+      - name: Configure git
+        run: |
+          git config user.name "diff-shades-gha"
+          git config user.email "diff-shades-gha@example.com"
+
+      - name: Build and install target revision
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+        run: |
+          ${{ matrix.target-setup-cmd }}
+          python -m pip install .
+
+      # Pull it from previous runs - we're NOT trying to get it from this run
+      # (but it wouldn't cause problems if we theoretically did)
+      - name: Attempt to find baseline analysis
+        id: baseline-cache
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ matrix.baseline-analysis }}
+          key: ${{ matrix.baseline-cache-key }}
+
+      - name: Analyze target revision (with repeated projects)
+        if: steps.baseline-cache.outputs.cache-hit == 'true'
+        run: |
+          diff-shades analyze ${{ matrix.target-analysis }} --work-dir projects-cache/ \
+          --force-${{ matrix.style }}-style -v \
+          --repeat-projects-from ${{ matrix.baseline-analysis }}
+
+      - name: Analyze target revision (without repeated projects)
+        if: steps.baseline-cache.outputs.cache-hit != 'true'
+        run:
+          diff-shades analyze ${{ matrix.target-analysis }} --work-dir projects-cache/
+          --force-${{ matrix.style }}-style -v
+
+      - name: Upload target analysis
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: ${{ matrix.target-analysis }}
+          path: ${{ matrix.target-analysis }}
+
+      - name: Check for failed files for target revision
+        run: diff-shades show-failed --check --show-log ${{ matrix.target-analysis }}
+
+  compare:
+    name: compare / ${{ matrix.mode }}
+    needs: ["configure", "analysis-base", "analysis-target"]
+    if: ${{ !cancelled() }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include: ${{ fromJson(needs.configure.outputs.matrix) }}
+
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
+
+      - uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
+        with:
+          merge-multiple: true
+
+      - name: Set up Python
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6.3.0
+        with:
+          python-version: "3.14"
+          pip-version: "25.3"
+          pip-install: --group diff-shades --group diff-shades-comment
+
+      - name: Generate HTML diff report
+        run: |
+          diff-shades --dump-html diff.html \
+          compare --diff ${{ matrix.baseline-analysis }} ${{ matrix.target-analysis }}
+
+      - name: Upload diff report
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: ${{ matrix.style }}-diff.html
+          path: diff.html
+
+      - name: Generate summary file (PR only)
+        if: github.event_name == 'pull_request'
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+        run: |
+          python scripts/diff_shades_gha_helper.py comment-body \
+          ${{ matrix.baseline-analysis }} ${{ matrix.target-analysis }} \
+          ${{ matrix.style }} ${{ matrix.mode }}
+
+      - name: Upload summary file (PR only)
+        if: github.event_name == 'pull_request'
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: .${{ matrix.style }}.pr-comment.md
+          path: .${{ matrix.style }}.pr-comment.md
+          include-hidden-files: true
+
+      - name: Verify zero changes (PR only)
+        if: matrix.mode == 'assert-no-changes'
+        run: |
+          diff-shades compare --check \
+          ${{ matrix.baseline-analysis }} ${{ matrix.target-analysis }} || \
+          (echo "Please verify you didn't change the stable code style unintentionally!" \
+          && exit 1)