feat: add multi-platform support for macOS and Windows

.github/workflows/_required.yml

@@ -99,15 +99,20 @@ jobs:
 
   pixi-check:
     name: pixi-check
-    runs-on: ubuntu-24.04
-    timeout-minutes: 10
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 15
     needs: lint
     permissions:
       contents: read
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-24.04, macos-14, macos-13, windows-2022]
     steps:
       - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10  # v6.0.3
       - name: Skip if no pixi.toml
         id: detect
+        shell: bash
         run: |
           if [ ! -f pixi.toml ]; then
             echo "::notice::No pixi.toml in repo, skipping pixi-check"
@@ -123,6 +128,7 @@ jobs:
           cache: true
       - name: pixi install (locked)
         if: steps.detect.outputs.skip == 'false'
+        shell: bash
         run: |
           if [ -f pixi.lock ]; then
             pixi install --locked

CONTRIBUTING.md

@@ -26,6 +26,8 @@ For an overview of the full ecosystem, see the
 - [Pixi](https://pixi.sh/) for environment management (installs Python 3.10+)
 - [Just](https://just.systems/) as the command runner
 
+**Supported platforms:** Linux (`linux-64`), macOS (`osx-64` Intel and `osx-arm64` Apple silicon), and Windows (`win-64`). All four are covered by CI; `pixi install` must succeed on each before merge.
+
 ### Environment Setup
 
 ```bash