macos package: derive package arch from the build target

The published v0.3.0 macOS installer is a structurally corrupt xar
archive: pkgutil and xar reject it even though its SHA-256 matches the
published checksum.

build-pkg.sh derived the architecture suffix in the .pkg filename from
`uname -m`. On the Apple-silicon macOS runner that always reports
arm64, so the cross-compiled x86_64 build also named its output
fips-<version>-macos-arm64.pkg. The release job downloads both build
artifacts with merge-multiple into one directory, where the two
identically named files collide and tear into a malformed result. The
x86_64 package never reaches the release at all.

Derive the package architecture from the Rust target triple, which is
authoritative for cross-compiles, instead of from the build host. Each
matrix leg now produces a distinctly named, arch-correct package, so
the two artifacts no longer collide.

Add a SHA-256 integrity chain so a corrupt or mismatched asset cannot
be published again:

- Capture the .pkg SHA-256 on the macOS runner, after the on-runner
  structural verification, into a sidecar file carried in the artifact.
- Add a verify-handoff job that runs on every trigger and asserts each
  downloaded .pkg still matches its macOS-runner SHA-256.
- Gate the release job on verify-handoff and repeat the check on the
  exact bytes about to be published.

The build step now asserts it produced the expected arch-named package
so a regression in the naming fails loudly rather than as a silent
collision.

Relates to #102. The published v0.3.0 macOS assets still need to be
rebuilt and reuploaded separately.

.github/workflows/package-macos.yml

@@ -100,14 +100,22 @@ jobs:
         shell: bash
         run: |
           : ${GITHUB_OUTPUT:=/tmp/github_output}
+          set -euo pipefail
 
-          PKG_FILE=$(find deploy -maxdepth 1 -type f -name "fips-*-macos-*.pkg" | sort | head -n 1)
-          if [[ -z "$PKG_FILE" ]]; then
-            echo "Missing macOS package" >&2
+          # build-pkg.sh names the package from the build target, so each
+          # matrix leg produces a distinctly named, arch-correct asset.
+          # Assert that here: a regression in that naming then fails loudly
+          # at the build stage instead of as a silent collision when the
+          # release job merges both artifacts into one directory.
+          EXPECTED="deploy/fips-${{ needs.determine-versioning.outputs.macos_package_version }}-macos-${{ matrix.arch }}.pkg"
+          if [[ ! -f "$EXPECTED" ]]; then
+            echo "Expected package $EXPECTED was not produced" >&2
+            echo "deploy/ contains:" >&2
+            ls -la deploy >&2 || true
             exit 1
           fi
 
-          echo "pkg=$PKG_FILE" >> "$GITHUB_OUTPUT"
+          echo "pkg=$EXPECTED" >> "$GITHUB_OUTPUT"
 
       - name: Verify .pkg structural correctness
         shell: bash
@@ -185,16 +193,27 @@ jobs:
           fi
           echo "==> .pkg verification PASSED"
 
-      - name: SHA-256 hash
+      - name: SHA-256 hash and sidecar
+        shell: bash
         run: |
+          set -euo pipefail
+          PKG="${{ steps.macos-assets.outputs.pkg }}"
           echo "==> macOS release asset:"
-          shasum -a 256 "${{ steps.macos-assets.outputs.pkg }}"
+          # Capture the SHA-256 of the verified .pkg on the macOS runner and
+          # write it to a sidecar file next to the .pkg, in the standard
+          # `<hash>  <basename>` shasum format. The verify-handoff and release
+          # jobs re-check the downloaded bytes against this value, so any
+          # corruption introduced after this point is detected before
+          # publication.
+          ( cd "$(dirname "$PKG")" && shasum -a 256 "$(basename "$PKG")" | tee "$(basename "$PKG").sha256" )
 
       - name: Upload artifact
         uses: actions/upload-artifact@v4
         with:
           name: fips_${{ needs.determine-versioning.outputs.macos_package_version }}_${{ matrix.arch }}_macos
-          path: ${{ steps.macos-assets.outputs.pkg }}
+          path: |
+            ${{ steps.macos-assets.outputs.pkg }}
+            ${{ steps.macos-assets.outputs.pkg }}.sha256
           retention-days: 30
 
       - name: Build summary
@@ -202,10 +221,61 @@ jobs:
           echo "Build Summary for macOS/${{ matrix.arch }}:"
           echo "  Package: ${{ steps.macos-assets.outputs.pkg }}"
 
+  verify-handoff:
+    name: Verify macOS package handoff integrity
+    runs-on: ubuntu-latest
+    needs: build
+
+    steps:
+      - name: Download macOS artifacts
+        uses: actions/download-artifact@v4
+        with:
+          path: dist
+          merge-multiple: true
+
+      - name: Verify .pkg integrity across the handoff
+        shell: bash
+        run: |
+          set -euo pipefail
+          cd dist
+
+          pkgs=$(find . -maxdepth 1 -type f -name '*.pkg' | LC_ALL=C sort)
+          if [[ -z "$pkgs" ]]; then
+            echo "FAIL: no .pkg artifacts were downloaded" >&2
+            exit 1
+          fi
+
+          fail=0
+          while IFS= read -r pkg; do
+            base=$(basename "$pkg")
+            sidecar="${pkg}.sha256"
+            if [[ ! -f "$sidecar" ]]; then
+              echo "FAIL: missing SHA-256 sidecar for $base" >&2
+              fail=1
+              continue
+            fi
+            expected=$(awk '{print $1}' "$sidecar")
+            actual=$(sha256sum "$pkg" | awk '{print $1}')
+            if [[ "$expected" != "$actual" ]]; then
+              echo "FAIL: $base SHA-256 mismatch across the artifact handoff" >&2
+              echo "  expected (macOS runner): $expected" >&2
+              echo "  actual   (downloaded):   $actual" >&2
+              fail=1
+              continue
+            fi
+            echo "PASS: $base matches the macOS-runner SHA-256 ($actual)"
+          done <<<"$pkgs"
+
+          if [[ "$fail" -ne 0 ]]; then
+            echo "==> macOS package handoff verification FAILED" >&2
+            exit 1
+          fi
+          echo "==> macOS package handoff verification PASSED"
+
   release:
     name: Publish macOS assets to GitHub Release
     runs-on: ubuntu-latest
-    needs: build
+    needs: [build, verify-handoff]
     if: startsWith(github.ref, 'refs/tags/')
     permissions:
       contents: write
@@ -217,6 +287,45 @@ jobs:
           path: dist
           merge-multiple: true
 
+      - name: Validate .pkg bytes before publishing
+        shell: bash
+        run: |
+          set -euo pipefail
+          cd dist
+
+          pkgs=$(find . -maxdepth 1 -type f -name '*.pkg' | LC_ALL=C sort)
+          if [[ -z "$pkgs" ]]; then
+            echo "FAIL: no .pkg artifacts were downloaded" >&2
+            exit 1
+          fi
+
+          fail=0
+          while IFS= read -r pkg; do
+            base=$(basename "$pkg")
+            sidecar="${pkg}.sha256"
+            if [[ ! -f "$sidecar" ]]; then
+              echo "FAIL: missing SHA-256 sidecar for $base" >&2
+              fail=1
+              continue
+            fi
+            expected=$(awk '{print $1}' "$sidecar")
+            actual=$(sha256sum "$pkg" | awk '{print $1}')
+            if [[ "$expected" != "$actual" ]]; then
+              echo "FAIL: $base SHA-256 mismatch on the bytes about to be published" >&2
+              echo "  expected (macOS runner): $expected" >&2
+              echo "  actual   (downloaded):   $actual" >&2
+              fail=1
+              continue
+            fi
+            echo "PASS: $base matches the macOS-runner SHA-256 ($actual)"
+          done <<<"$pkgs"
+
+          if [[ "$fail" -ne 0 ]]; then
+            echo "==> pre-publish .pkg verification FAILED; not publishing" >&2
+            exit 1
+          fi
+          echo "==> pre-publish .pkg verification PASSED"
+
       - name: Generate macOS release checksums
         run: |
           cd dist