ci: add cosign keyless sigs, SBOM, and provenance attestation

Sign each ISO chunk with cosign keyless OIDC, generate an SPDX SBOM
of the build output, and attach an in-toto build-provenance
attestation. Sigs/certs/SBOM are uploaded alongside the ISO parts in
the ci-latest rolling prerelease so the test/auto-install.sh path
can verify before reassembling.

Action versions are major-version tags (@v3, @v0, @v2). SHA-pinning
is tracked separately to keep this PR small and avoid the long web
lookups that stalled the previous attempt.

.github/workflows/build-iso.yml

@@ -20,7 +20,9 @@ on:
     types: [published]
 
 permissions:
-  contents: write   # needed for action-gh-release to create+update ci-latest
+  contents: write       # needed for action-gh-release to create+update ci-latest
+  id-token: write       # cosign keyless OIDC + attest-build-provenance
+  attestations: write   # attest-build-provenance writes the attestation
 
 jobs:
   build:
@@ -197,6 +199,34 @@ jobs:
           echo "[OK] split into:"
           ls "${ISO}".part-*
 
+      - name: Install cosign
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
+        uses: sigstore/cosign-installer@v3
+
+      - name: Sign ISO parts (keyless)
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
+        run: |
+          cd build/out
+          for f in *.part-*; do
+            cosign sign-blob --yes "$f" \
+              --output-signature "$f.sig" \
+              --output-certificate "$f.pem"
+          done
+
+      - name: Generate SBOM (SPDX)
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
+        uses: anchore/sbom-action@v0
+        with:
+          path: build/out
+          format: spdx-json
+          output-file: build/out/veilor-os.spdx.json
+
+      - name: Build provenance attestation
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
+        uses: actions/attest-build-provenance@v2
+        with:
+          subject-path: 'build/out/*.iso.part-*'
+
       - name: Publish to ci-latest rolling prerelease
         if: success() && github.ref == 'refs/heads/main'
         uses: softprops/action-gh-release@v2
@@ -220,6 +250,9 @@ jobs:
           files: |
             build/out/*.iso.part-*
             build/out/*.sha256
+            build/out/*.sig
+            build/out/*.pem
+            build/out/*.spdx.json
 
       # Build log on failure: print inline + skip artifact upload to avoid
       # quota wall. Job log retains everything anyway.