ci(macos): run the darwin target this repository already ships

.github/workflows/macos.yml

@@ -0,0 +1,55 @@
+# macOS coverage for a target this repository already ships.
+#
+# `schemas/v1/release-installation.schema.json` and `bundle-trust.schema.json`
+# both declare `target_os: {"enum": ["darwin", "linux"]}`, and the release
+# builder emits darwin artifacts. Until now every test ran on ubuntu only, so
+# darwin was cross-compiled and never executed. Cross-compilation proves it
+# builds; it says nothing about behaviour, and this engine is full of the things
+# that differ between the two — path resolution, os.Root confinement, file
+# locking, temp directory semantics, case sensitivity.
+#
+# This is a separate workflow rather than a matrix inside `gds-ci.yml` because
+# that file is a generated projection with a single `.Runner`; widening it is a
+# change to the engine's template and generator plus a governed regeneration.
+# `.github` is outside `developmentBundleSourcePaths`, so adding a file here
+# does not disturb the bundle lock — the same reason `release-bundle.yml` and
+# `codeql.yml` sit beside the generated workflow.
+#
+# Standard macOS runners are unmetered on public repositories. macOS-XL is not,
+# and is deliberately not used.
+name: macos
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions: {}
+
+concurrency:
+  group: macos-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
+jobs:
+  darwin:
+    name: darwin
+    runs-on: macos-latest
+    timeout-minutes: 20
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - uses: actions/setup-go@b7ad1dad31e06c5925ef5d2fc7ad053ef454303e # v7.0.0
+        with:
+          go-version-file: go.mod
+          cache: true
+      # The repository's own declared commands, from `.gds/repository.yaml`,
+      # so this lane cannot drift into testing something else.
+      - name: go vet
+        run: go vet ./...
+      - name: go build
+        run: go build -trimpath ./core/cmd/gds
+      - name: go test
+        run: go test ./...