ci: dedupe workflow via composite setup action and linux test matrix

GitHub Actions doesn't support YAML anchors, so use its native DRY
mechanisms instead:
- Add a local composite action (.github/actions/setup) for the uv
  install step, used by all jobs. (checkout must stay per-job: a local
  action requires the repo to already be checked out.)
- Collapse the 4 linux-* test jobs into a single `linux-tests` job with a
  strategy.matrix over {highest,lowest} x {slow,fast}, so needs:/env:/steps
  are declared once.
- Drop the redundant "Stage 2 runs only if..." comment.

Author: guru-desh

.github/actions/setup/action.yml

@@ -0,0 +1,10 @@
+name: Setup
+description: Install uv (cache disabled) for coreai-optimization CI jobs. Assumes the repository is already checked out.
+
+runs:
+  using: composite
+  steps:
+    - name: Install uv
+      uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+      with:
+        enable-cache: false

.github/workflows/ci.yaml

@@ -21,7 +21,6 @@ concurrency:
 jobs:
 
   # ── Stage 1: cheap correctness gate (ubuntu-latest, ungated). ──
-  # Stage 2 runs only if all three of these pass.
   check:
     name: Linux / make check
     runs-on: ubuntu-latest
@@ -31,10 +30,7 @@ jobs:
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
         with:
           fetch-depth: 0
-      - name: Install uv
-        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
-        with:
-          enable-cache: false
+      - uses: ./.github/actions/setup
       - name: Run `make check`
         run: make check
 
@@ -47,10 +43,7 @@ jobs:
     steps:
       - name: Check out repository
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
-      - name: Install uv
-        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
-        with:
-          enable-cache: false
+      - uses: ./.github/actions/setup
       - name: Run `make test-smoke`
         run: make test-smoke
 
@@ -63,10 +56,7 @@ jobs:
     steps:
       - name: Check out repository
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
-      - name: Install uv
-        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
-        with:
-          enable-cache: false
+      - uses: ./.github/actions/setup
       - name: Run `make test-tutorials`
         run: make test-tutorials
 
@@ -82,10 +72,7 @@ jobs:
     steps:
       - name: Check out repository
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
-      - name: Install uv
-        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
-        with:
-          enable-cache: false
+      - uses: ./.github/actions/setup
       - name: Run `make test-highest-pytorch`
         run: make test-highest-pytorch PYTEST_ARGS="--marker 'not slow' --junit"
       - name: Upload test results
@@ -95,94 +82,27 @@ jobs:
           name: test-results-macos-highest-fast
           path: test-results/
 
-  linux-highest-slow:
-    name: Linux / make test-highest-pytorch / markers=slow
+  linux-tests:
+    name: Linux / make test-${{ matrix.torch }}-pytorch / markers=${{ matrix.speed == 'slow' && 'slow' || 'not-slow' }}
     needs: [check, test-smoke, test-tutorials]
     runs-on: ubuntu-latest
     timeout-minutes: 120
     env:
       INSTALL_PRECOMMIT: "false"
+    strategy:
+      fail-fast: false
+      matrix:
+        torch: [highest, lowest]
+        speed: [slow, fast]
     steps:
       - name: Check out repository
         uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
-      - name: Install uv
-        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
-        with:
-          enable-cache: false
-      - name: Run `make test-highest-pytorch`
-        run: make test-highest-pytorch PYTEST_ARGS="--marker slow --junit"
-      - name: Upload test results
-        if: always()
-        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
-        with:
-          name: test-results-linux-highest-slow
-          path: test-results/
-
-  linux-highest-fast:
-    name: Linux / make test-highest-pytorch / markers=not-slow
-    needs: [check, test-smoke, test-tutorials]
-    runs-on: ubuntu-latest
-    timeout-minutes: 120
-    env:
-      INSTALL_PRECOMMIT: "false"
-    steps:
-      - name: Check out repository
-        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
-      - name: Install uv
-        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
-        with:
-          enable-cache: false
-      - name: Run `make test-highest-pytorch`
-        run: make test-highest-pytorch PYTEST_ARGS="--marker 'not slow' --junit"
-      - name: Upload test results
-        if: always()
-        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
-        with:
-          name: test-results-linux-highest-fast
-          path: test-results/
-
-  linux-lowest-slow:
-    name: Linux / make test-lowest-pytorch / markers=slow
-    needs: [check, test-smoke, test-tutorials]
-    runs-on: ubuntu-latest
-    timeout-minutes: 120
-    env:
-      INSTALL_PRECOMMIT: "false"
-    steps:
-      - name: Check out repository
-        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
-      - name: Install uv
-        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
-        with:
-          enable-cache: false
-      - name: Run `make test-lowest-pytorch`
-        run: make test-lowest-pytorch PYTEST_ARGS="--marker slow --junit"
-      - name: Upload test results
-        if: always()
-        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
-        with:
-          name: test-results-linux-lowest-slow
-          path: test-results/
-
-  linux-lowest-fast:
-    name: Linux / make test-lowest-pytorch / markers=not-slow
-    needs: [check, test-smoke, test-tutorials]
-    runs-on: ubuntu-latest
-    timeout-minutes: 120
-    env:
-      INSTALL_PRECOMMIT: "false"
-    steps:
-      - name: Check out repository
-        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
-      - name: Install uv
-        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
-        with:
-          enable-cache: false
-      - name: Run `make test-lowest-pytorch`
-        run: make test-lowest-pytorch PYTEST_ARGS="--marker 'not slow' --junit"
+      - uses: ./.github/actions/setup
+      - name: Run `make test-${{ matrix.torch }}-pytorch`
+        run: make test-${{ matrix.torch }}-pytorch PYTEST_ARGS="--marker '${{ matrix.speed == 'slow' && 'slow' || 'not slow' }}' --junit"
       - name: Upload test results
         if: always()
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          name: test-results-linux-lowest-fast
+          name: test-results-linux-${{ matrix.torch }}-${{ matrix.speed }}
           path: test-results/