feat(FN-4646): complete Step 4 — prefer landed file metadata in dashboard

Fusion-Task-Id: FN-4646
Fusion-Task-Lineage: f921098e-3407-40fa-984d-720e0c3248c7

packages/dashboard/app/components/__tests__/TaskCard.test.tsx

@@ -2444,6 +2444,31 @@ describe("TaskCard", () => {
     expect(filesChangedButton).toBeNull();
   });
 
+  it("prefers landedFiles fallback label for done tasks when lineage stats are unavailable", () => {
+    const onOpenDetailWithTab = vi.fn();
+    useTaskDiffStatsMock.mockReturnValue({ stats: null, loading: false });
+
+    render(
+      <TaskCard
+        task={makeTask({
+          column: "done",
+          mergeDetails: { landedFiles: ["a.ts", "b.ts"] },
+          modifiedFiles: ["a.ts", "b.ts", "c.ts", "d.ts", "e.ts", "f.ts"],
+        })}
+        onOpenDetail={noop}
+        addToast={noop}
+        onOpenDetailWithTab={onOpenDetailWithTab}
+      />,
+    );
+
+    const landedButton = screen.getByRole("button", { name: "2 files in merged commit" });
+    expect(landedButton).toBeDefined();
+
+    fireEvent.click(landedButton);
+    expect(onOpenDetailWithTab).toHaveBeenCalledTimes(1);
+    expect(onOpenDetailWithTab.mock.calls[0]?.[1]).toBe("changes");
+  });
+
   it("shows execution-touched fallback label for done tasks when lineage stats are unavailable", () => {
     const onOpenDetailWithTab = vi.fn();
     useTaskDiffStatsMock.mockReturnValue({ stats: null, loading: false });

packages/dashboard/app/components/__tests__/TaskChangesTab.test.tsx

@@ -570,7 +570,31 @@ describe("TaskChangesTab — regression: non-done tasks and done-without-commitS
     expect(mockFetchTaskDiff).toHaveBeenCalledWith("FN-001", undefined, undefined);
   });
 
-  it("done task without commitSha falls back to modifiedFiles when available", async () => {
+  it("done task without commitSha prefers landedFiles over stale modifiedFiles", async () => {
+    mockFetchTaskDiff.mockResolvedValue({ files: [], stats: { filesChanged: 0, additions: 0, deletions: 0 } });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={{ filesChanged: 0, insertions: 0, deletions: 0, landedFiles: ["packages/cli/src/commands/__tests__/settings.test.ts"] }}
+        modifiedFiles={["packages/cli/src/commands/__tests__/settings.test.ts", "packages/cli/src/commands/__tests__/task.test.ts"]}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("1 file in the merged commit.")).toBeTruthy();
+    });
+    expect(screen.getByText("These are files captured from the merged commit metadata. The lineage-backed diff is unavailable for this task.")).toBeTruthy();
+    expect(screen.getByText("packages/cli/src/commands/__tests__/settings.test.ts")).toBeTruthy();
+    expect(screen.queryByText("packages/cli/src/commands/__tests__/task.test.ts")).toBeNull();
+    expect(screen.queryByText(/Error loading changes:/)).toBeNull();
+    expect(screen.queryByText("Files Changed (2)")).toBeNull();
+    expect(mockFetchTaskDiff).toHaveBeenCalledWith("FN-001", undefined, undefined);
+  });
+
+  it("done task without commitSha falls back to modifiedFiles when landedFiles unavailable", async () => {
     mockFetchTaskDiff.mockResolvedValue({ files: [], stats: { filesChanged: 0, additions: 0, deletions: 0 } });
 
     render(