fix: select rollback artifact pairs atomically

tests/unit/release-coherence.test.ts

@@ -100,8 +100,10 @@ async function releaseV2With(
 
 function artifactReader(
   entries: Readonly<Record<string, unknown | Error>>,
+  reads: string[] = [],
 ): (path: string) => Promise<unknown> {
   return async (path) => {
+    reads.push(path);
     const value = entries[path];
     if (value === undefined) {
       throw Object.assign(new Error(`missing ${path}`), { code: "ENOENT" });
@@ -315,11 +317,54 @@ describe("release coherence", () => {
       }),
     ).resolves.toMatchObject({ coherence: { compatible: false } });
 
+    const releaseOnlyMissingReads: string[] = [];
     await expect(
       verifyRollbackReleaseCoherence({
-        readArtifact: artifactReader(validPublic),
+        readArtifact: artifactReader(
+          {
+            "dist/config.json": runtimeV2,
+            ...validPublic,
+          },
+          releaseOnlyMissingReads,
+        ),
+        contractPackages,
+      }),
+    ).rejects.toThrow("primary rollback artifact pair");
+    expect(releaseOnlyMissingReads).toEqual([
+      "dist/release-manifest.json",
+      "dist/config.json",
+    ]);
+
+    const runtimeOnlyMissingReads: string[] = [];
+    await expect(
+      verifyRollbackReleaseCoherence({
+        readArtifact: artifactReader(
+          {
+            "dist/release-manifest.json": exactV2,
+            ...validPublic,
+          },
+          runtimeOnlyMissingReads,
+        ),
+        contractPackages,
+      }),
+    ).rejects.toThrow("primary rollback artifact pair");
+    expect(runtimeOnlyMissingReads).toEqual([
+      "dist/release-manifest.json",
+      "dist/config.json",
+    ]);
+
+    const missingPairReads: string[] = [];
+    await expect(
+      verifyRollbackReleaseCoherence({
+        readArtifact: artifactReader(validPublic, missingPairReads),
         contractPackages,
       }),
     ).resolves.toMatchObject({ coherence: { compatible: true } });
+    expect(missingPairReads).toEqual([
+      "dist/release-manifest.json",
+      "dist/config.json",
+      "public/release-manifest.json",
+      "public/config.json",
+    ]);
   });
 });