feat(sign): attest only the platforms a release adds

netork's release joins an arm64 image to the amd64 one its dev build
already signed and attested, under a new index. ATTEST_PLATFORMS (the sign
action's attest-platforms) signs the new index and every platform behind
it, and attests only the ones named.

tests/test_signing.py

@@ -120,6 +120,30 @@ class TestSign:
         assert sum(f"@{AMD}" in ln for ln in verified) == 2
         assert sum(f"@{ARM}" in ln for ln in verified) == 2
 
+    def test_a_release_index_attests_only_the_platform_it_added(self, tmp_path):
+        """netork's release joins arm64 to the amd64 image its dev build already
+        signed and attested: the new index is signed throughout, and only arm64
+        needs its CIS result and SBOM (ATTEST_PLATFORMS)."""
+        raw = {
+            "mediaType": "application/vnd.oci.image.index.v1+json",
+            "manifests": [
+                {"digest": AMD, "platform": {"os": "linux", "architecture": "amd64"}},
+                {"digest": ARM, "platform": {"os": "linux", "architecture": "arm64"}},
+            ],
+        }
+        env = self._env(tmp_path, raw, {}, ATTEST_PLATFORMS="arm64")
+        attest = self._attest_dir(tmp_path, "arm64")
+
+        result = _run(SIGN, f"{IMAGE}:0.41.0@{INDEX}", str(attest), env=env)
+
+        assert result.returncode == 0, result.stderr
+        log = _log(tmp_path)
+        assert any("sign --yes --recursive" in ln and f"@{INDEX}" in ln for ln in log)
+        attests = [ln for ln in log if " attest " in ln]
+        assert len(attests) == 2 and all(f"@{ARM}" in ln for ln in attests)
+        assert f"signed and attested: {IMAGE}@{AMD}" not in result.stdout
+        assert f"signed: {IMAGE}@{AMD} (amd64)" in result.stdout
+
     def test_a_missing_expected_platform_stops_it(self, tmp_path):
         env = self._env(
             tmp_path, {"mediaType": "x"}, {"architecture": "amd64"}, EXPECT_PLATFORMS="amd64 arm64"