test(FN-4433): complete Step 3 — cover file-scope violation audit emission

Fusion-Task-Id: FN-4433
Fusion-Task-Lineage: 63ee20ca-1e04-4b42-b386-a1e586aac889

packages/engine/src/__tests__/merger-file-scope-invariant.test.ts

@@ -6,6 +6,7 @@ import {
   assertSquashOverlapsFileScope,
   attemptWithSideStrategy,
   commitOrAmendMergeWithFixes,
+  enforceSquashFileScopeInvariant,
   executeMergeAttempt,
   FileScopeViolationError,
 } from "../merger.js";
@@ -195,6 +196,108 @@ describe("assertSquashOverlapsFileScope", () => {
   });
 });
 
+describe("enforceSquashFileScopeInvariant audit emission", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("emits run_audit event on file-scope violation", async () => {
+    const store = createInvariantStore(["packages/engine/src/merger.ts"]);
+    const auditor = { git: vi.fn().mockResolvedValue(undefined) };
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr === "git diff --cached --name-only") return "packages/core/src/store.ts";
+      if (cmdStr === "git reset --merge") return "";
+      return "";
+    });
+
+    await expect(enforceSquashFileScopeInvariant({
+      store: store as never,
+      taskId: "FN-4073",
+      rootDir: "/tmp/root",
+      task: await (store as any).getTask("FN-4073"),
+      resetLabel: "file-scope invariant violation",
+      auditor: auditor as any,
+    })).rejects.toBeInstanceOf(FileScopeViolationError);
+
+    expect(auditor.git).toHaveBeenCalledTimes(1);
+    expect(auditor.git).toHaveBeenCalledWith({
+      type: "merge:file-scope-violation",
+      target: "FN-4073",
+      metadata: {
+        resetLabel: "file-scope invariant violation",
+        stagedFiles: ["packages/core/src/store.ts"],
+        declaredScope: ["packages/engine/src/merger.ts"],
+        stagedFileCount: 1,
+        declaredScopeCount: 1,
+      },
+    });
+  });
+
+  it("does not emit when scopeOverride bypasses invariant", async () => {
+    const store = createInvariantStore(["packages/engine/src/merger.ts"], { scopeOverride: true });
+    const auditor = { git: vi.fn().mockResolvedValue(undefined) };
+    mockedExecSync.mockImplementation(() => "packages/core/src/store.ts");
+
+    await expect(enforceSquashFileScopeInvariant({
+      store: store as never,
+      taskId: "FN-4073",
+      rootDir: "/tmp/root",
+      task: await (store as any).getTask("FN-4073"),
+      resetLabel: "file-scope invariant violation",
+      auditor: auditor as any,
+    })).resolves.toBeUndefined();
+
+    expect(auditor.git).not.toHaveBeenCalled();
+  });
+
+  it("does not mask original violation when audit emission fails", async () => {
+    const store = createInvariantStore(["packages/engine/src/merger.ts"]);
+    const auditor = { git: vi.fn().mockRejectedValue(new Error("audit boom")) };
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr === "git diff --cached --name-only") return "packages/core/src/store.ts";
+      if (cmdStr === "git reset --merge") return "";
+      return "";
+    });
+
+    await expect(enforceSquashFileScopeInvariant({
+      store: store as never,
+      taskId: "FN-4073",
+      rootDir: "/tmp/root",
+      task: await (store as any).getTask("FN-4073"),
+      resetLabel: "file-scope invariant violation",
+      auditor: auditor as any,
+    })).rejects.toBeInstanceOf(FileScopeViolationError);
+
+    expect(store.appendAgentLog).toHaveBeenCalledWith(
+      "FN-4073",
+      expect.stringContaining("File-scope invariant violation"),
+      "tool_error",
+      expect.stringContaining("declaredScope:"),
+      "merger",
+    );
+  });
+
+  it("keeps backward compatibility when auditor is omitted", async () => {
+    const store = createInvariantStore(["packages/engine/src/merger.ts"]);
+    mockedExecSync.mockImplementation((cmd: any) => {
+      const cmdStr = String(cmd);
+      if (cmdStr === "git diff --cached --name-only") return "packages/core/src/store.ts";
+      if (cmdStr === "git reset --merge") return "";
+      return "";
+    });
+
+    await expect(enforceSquashFileScopeInvariant({
+      store: store as never,
+      taskId: "FN-4073",
+      rootDir: "/tmp/root",
+      task: await (store as any).getTask("FN-4073"),
+      resetLabel: "file-scope invariant violation",
+    })).rejects.toBeInstanceOf(FileScopeViolationError);
+  });
+});
+
 describe("file-scope invariant wiring", () => {
   beforeEach(() => {
     vi.clearAllMocks();

packages/engine/src/merger.ts

@@ -3171,6 +3171,7 @@ export async function commitOrAmendMergeWithFixes(
   aiSubject?: string | null,
   fixModifiedFiles: ReadonlySet<string> = new Set(),
   store?: TaskStore,
+  auditor?: RunAuditor,
 ): Promise<MergeFinalizeResult> {
   try {
     // Build an allowlist of paths we are permitted to stage.
@@ -3517,7 +3518,7 @@ export async function commitOrAmendMergeWithFixes(
           rootDir,
           task: await store.getTask(taskId),
           resetLabel: "file-scope invariant violation",
-          auditor: audit,
+          auditor,
         });
       }
       await runDiffVolumeGate({
@@ -3560,7 +3561,7 @@ export async function commitOrAmendMergeWithFixes(
         rootDir,
         task: await store.getTask(taskId),
         resetLabel: "file-scope invariant violation",
-        auditor: audit,
+        auditor,
       });
     }
     await runDiffVolumeGate({
@@ -3770,7 +3771,7 @@ export function formatFileScopeViolationAgentLog(error: FileScopeViolationError)
   ].join("\n");
 }
 
-async function enforceSquashFileScopeInvariant(params: {
+export async function enforceSquashFileScopeInvariant(params: {
   store: TaskStore;
   taskId: string;
   rootDir: string;
@@ -6669,6 +6670,7 @@ export async function aiMergeTask(
                 aiMergeSubject,
                 verificationFixModifiedFiles,
                 store,
+                audit,
               );
               if (!finalized.ok) {
                 // Phantom-merge guard: refused to fabricate a commit. Reset
@@ -6791,6 +6793,7 @@ export async function aiMergeTask(
               aiMergeSubject,
               buildFixModifiedFiles,
               store,
+              audit,
             );
             if (!finalized.ok) {
               // Phantom-merge guard: the verification fix passed but no