feat(FN-3998): add task lineage commit associations API, UI, and tests

Adds task lineage commit associations: a new API route stores and exposes which commits belong to which task, the `TaskChangesTab` surfaces these lineage links visually, and documentation covers the reconciliation model. Includes comprehensive tests for both the route and component.

Fusion-Task-Id: FN-3998

packages/dashboard/src/__tests__/routes-task-commit-associations.test.ts

@@ -0,0 +1,149 @@
+import { describe, it, expect } from "vitest";
+import { EventEmitter } from "node:events";
+import type { Task, TaskCommitAssociation } from "@fusion/core";
+import { createServer } from "../server.js";
+
+class MockStore extends EventEmitter {
+  private tasks = new Map<string, Task>();
+  private associations = new Map<string, TaskCommitAssociation[]>();
+
+  getRootDir(): string {
+    return "/tmp/fn-3998";
+  }
+
+  getFusionDir(): string {
+    return "/tmp/fn-3998/.fusion";
+  }
+
+  getDatabase() {
+    return {
+      exec: () => undefined,
+      prepare: () => ({ run: () => ({ changes: 0 }), get: () => undefined, all: () => [] }),
+    };
+  }
+
+  getMissionStore() {
+    return {
+      listMissions: async () => [],
+      createMission: () => undefined,
+      getMission: () => undefined,
+      updateMission: () => undefined,
+      deleteMission: () => undefined,
+      listTemplates: async () => [],
+      createTemplate: () => undefined,
+      getTemplate: () => undefined,
+      updateTemplate: () => undefined,
+      deleteTemplate: () => undefined,
+      instantiateMission: () => undefined,
+    };
+  }
+
+  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, rows: TaskCommitAssociation[]): void {
+    this.associations.set(lineageId, rows);
+  }
+
+  async getTaskCommitAssociationsByLineageId(lineageId: string): Promise<TaskCommitAssociation[]> {
+    return this.associations.get(lineageId) ?? [];
+  }
+}
+
+function createTask(overrides: Partial<Task> = {}): Task {
+  return {
+    id: "FN-3998",
+    title: "Lineage task",
+    description: "Test",
+    column: "done",
+    dependencies: [],
+    steps: [],
+    currentStep: 0,
+    log: [],
+    createdAt: "2026-05-11T00:00:00.000Z",
+    updatedAt: "2026-05-11T00:00:00.000Z",
+    columnMovedAt: "2026-05-11T00:00:00.000Z",
+    lineageId: "lineage-1",
+    ...overrides,
+  };
+}
+
+async function getCommitAssociations(
+  app: Parameters<typeof import("../test-request.js").get>[0],
+  taskId: string,
+): Promise<{ status: number; body: any }> {
+  const { get } = await import("../test-request.js");
+  return get(app, `/api/tasks/${taskId}/commit-associations`);
+}
+
+describe("GET /api/tasks/:id/commit-associations", () => {
+  it("returns 404 when task is unknown", async () => {
+    const app = createServer(new MockStore() as any);
+    const response = await getCommitAssociations(app, "FN-UNKNOWN");
+    expect(response.status).toBe(404);
+    expect(response.body).toEqual({ error: "Task not found" });
+  });
+
+  it("returns lineage associations for a known task", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ id: "FN-2000", lineageId: "lineage-2000" }));
+    store.setAssociations("lineage-2000", [
+      {
+        id: "assoc-1",
+        taskLineageId: "lineage-2000",
+        taskIdSnapshot: "FN-2000",
+        commitSha: "abc1234def",
+        commitSubject: "feat(FN-2000): add lineage API",
+        authoredAt: "2026-05-11T02:00:00.000Z",
+        matchedBy: "canonical-lineage-trailer",
+        confidence: "canonical",
+        note: "primary commit",
+        createdAt: "2026-05-11T02:01:00.000Z",
+        updatedAt: "2026-05-11T02:01:00.000Z",
+      },
+    ]);
+
+    const app = createServer(store as any);
+    const response = await getCommitAssociations(app, "FN-2000");
+
+    expect(response.status).toBe(200);
+    expect(response.body).toEqual({
+      taskId: "FN-2000",
+      lineageId: "lineage-2000",
+      associations: [
+        {
+          commitSha: "abc1234def",
+          commitSubject: "feat(FN-2000): add lineage API",
+          authoredAt: "2026-05-11T02:00:00.000Z",
+          matchedBy: "canonical-lineage-trailer",
+          confidence: "canonical",
+          taskIdSnapshot: "FN-2000",
+          note: "primary commit",
+        },
+      ],
+    });
+  });
+
+  it("returns empty associations for known task with no rows", async () => {
+    const store = new MockStore();
+    store.addTask(createTask({ id: "FN-2001", lineageId: "lineage-2001" }));
+    const app = createServer(store as any);
+
+    const response = await getCommitAssociations(app, "FN-2001");
+    expect(response.status).toBe(200);
+    expect(response.body).toEqual({
+      taskId: "FN-2001",
+      lineageId: "lineage-2001",
+      associations: [],
+    });
+  });
+});