feat(FN-4521): complete Step 2 — fix done lineage aggregation

Fusion-Task-Id: FN-4521
Fusion-Task-Lineage: 675472a2-480d-4b9e-8861-98d2d82680db

packages/dashboard/src/__tests__/routes-diff-lineage.test.ts

@@ -0,0 +1,148 @@
+import { describe, it, expect } from "vitest";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { execFileSync } from "node:child_process";
+import type { Task, TaskCommitAssociation } from "@fusion/core";
+import { EventEmitter } from "node:events";
+import { createServer } from "../server.js";
+
+class RealGitStore extends EventEmitter {
+  private tasks = new Map<string, Task>();
+  private associations = new Map<string, TaskCommitAssociation[]>();
+
+  constructor(private rootDir: string) {
+    super();
+  }
+
+  getRootDir(): string {
+    return this.rootDir;
+  }
+
+  getFusionDir(): string {
+    return join(this.rootDir, ".fusion");
+  }
+
+  getDatabase() {
+    return {
+      exec: () => {},
+      prepare: () => ({ run: () => ({ changes: 0 }), get: () => undefined, all: () => [] }),
+    };
+  }
+
+  getMissionStore() {
+    return {
+      listMissions: async () => [],
+      listTemplates: async () => [],
+    };
+  }
+
+  async listTasks(): Promise<Task[]> {
+    return Array.from(this.tasks.values());
+  }
+
+  getTask(id: string): Task | undefined {
+    return this.tasks.get(id);
+  }
+
+  addTask(task: Task): void {
+    this.tasks.set(task.id, task);
+  }
+
+  setAssociations(lineageId: string, associations: TaskCommitAssociation[]): void {
+    this.associations.set(lineageId, associations);
+  }
+
+  async getTaskCommitAssociationsByLineageId(lineageId: string): Promise<TaskCommitAssociation[]> {
+    return this.associations.get(lineageId) ?? [];
+  }
+}
+
+function git(cwd: string, ...args: string[]): string {
+  return execFileSync("git", args, { cwd, encoding: "utf8" }).trim();
+}
+
+function commitFile(cwd: string, file: string, content: string, message: string): string {
+  writeFileSync(join(cwd, file), content);
+  git(cwd, "add", file);
+  git(cwd, "commit", "-m", message);
+  return git(cwd, "rev-parse", "HEAD");
+}
+
+describe("FN-4521 done-task lineage aggregation", () => {
+  it("keeps lineage-only files and excludes interleaved non-lineage commits", async () => {
+    const rootDir = mkdtempSync(join(tmpdir(), "fn-4521-lineage-"));
+
+    try {
+      git(rootDir, "init", "-b", "main");
+      git(rootDir, "config", "user.email", "fusion@example.com");
+      git(rootDir, "config", "user.name", "Fusion");
+
+      commitFile(rootDir, "base.txt", "base\n", "A base");
+
+      git(rootDir, "checkout", "-b", "task-branch");
+      const commitB = commitFile(rootDir, "a.ts", "export const a = 1;\n", "B task change a");
+
+      git(rootDir, "checkout", "main");
+      commitFile(rootDir, "unrelated.ts", "export const unrelated = true;\n", "C foreign change");
+
+      git(rootDir, "checkout", "task-branch");
+      git(rootDir, "merge", "main", "--no-edit");
+      const commitD = commitFile(rootDir, "b.ts", "export const b = 2;\n", "D task change b");
+
+      git(rootDir, "checkout", "main");
+      git(rootDir, "merge", "task-branch", "--no-ff", "-m", "M merge task branch");
+      const mergeCommit = git(rootDir, "rev-parse", "HEAD");
+
+      const lineageId = "lin-fn-4521";
+      const store = new RealGitStore(rootDir);
+      store.addTask({
+        id: "FN-4521",
+        title: "lineage test",
+        description: "lineage test",
+        column: "done",
+        dependencies: [],
+        steps: [],
+        currentStep: 0,
+        log: [],
+        createdAt: "2026-05-14T00:00:00.000Z",
+        updatedAt: "2026-05-14T00:00:00.000Z",
+        columnMovedAt: "2026-05-14T00:00:00.000Z",
+        lineageId,
+        baseBranch: "main",
+        mergeDetails: { commitSha: mergeCommit, filesChanged: 2 },
+      } as Task);
+
+      const mkAssoc = (sha: string, authoredAt: string): TaskCommitAssociation => ({
+        lineageId,
+        commitSha: sha,
+        commitSubject: sha,
+        authoredAt,
+        matchedBy: "manual",
+        confidence: 1,
+        taskIdSnapshot: "FN-4521",
+        note: null,
+        createdAt: authoredAt,
+        updatedAt: authoredAt,
+      });
+
+      store.setAssociations(lineageId, [
+        mkAssoc(commitB, "2026-05-14T00:00:01.000Z"),
+        mkAssoc(commitD, "2026-05-14T00:00:02.000Z"),
+      ]);
+
+      const app = createServer(store as any);
+      const { get } = await import("../test-request.js");
+      const response = await get(app, "/api/tasks/FN-4521/diff");
+
+      expect(response.status).toBe(200);
+      const paths = response.body.files.map((f: { path: string }) => f.path).sort();
+      expect(paths).toContain("a.ts");
+      expect(paths).toContain("b.ts");
+      // FN-4521 regression: pre-fix netRange (B^..merge) swept in unrelated.ts from interleaved commit C.
+      expect(paths).not.toContain("unrelated.ts");
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+});

packages/dashboard/src/__tests__/routes-diff.test.ts

@@ -304,7 +304,7 @@ describe("FN-4308 multi-commit done task aggregation", () => {
     expect(response.body.stats.filesChanged).toBe(2);
   });
 
-  it("falls back to commitSha enumeration when aggregation under-counts mergeDetails", async () => {
+  it("keeps lineage aggregation when mergeDetails filesChanged is higher", async () => {
     const store = new MockStore();
     store.addTask(createTask({ column: "done", lineageId: "lin-1", mergeDetails: { commitSha: "merge", filesChanged: 3 } }));
     store.setAssociations("lin-1", [makeAssociation("assoc", "2026-04-01T00:00:00.000Z")]);
@@ -335,7 +335,7 @@ describe("FN-4308 multi-commit done task aggregation", () => {
     const app = createServer(store as any);
     const response = await requestDiff(app);
     expect(response.status).toBe(200);
-    expect(response.body.files.map((f: any) => f.path).sort()).toEqual(["one.ts", "three.ts", "two.ts"]);
+    expect(response.body.files.map((f: any) => f.path).sort()).toEqual(["a.txt", "b.txt"]);
     expect(response.body.files.length).toBe(response.body.stats.filesChanged);
   });