feat(FN-3979): validate overlap coverage for FN-3834 and keep tests type-sa

FN-3979 adds test coverage validating the FN-3834 overlap detection fix: Step 1 introduces overlap validation tests in the merger and self-healing test suites, and Step 2 cleans up a type-safety issue in the overlap test.

Fusion-Task-Id: FN-3979

packages/engine/src/__tests__/merger.test.ts

@@ -1085,10 +1085,11 @@ describe("aiMergeTask — merge-target branch resolution", () => {
 });
 
 describe("aiMergeTask — no-op short-circuit", () => {
-  it("finalizes to done when branch has zero commits ahead of base", async () => {
+  it("finalizes to done when branch has zero commits ahead of base (including review-level-0 coordination tasks)", async () => {
     const store = createMockStore({
       id: "FN-3834-NOOP",
       branch: "fusion/fn-3834-noop",
+      reviewLevel: 0,
       mergeDetails: { mergeTargetBranch: "main" },
       worktree: "/tmp/root/.worktrees/FN-3834-NOOP",
     });

packages/engine/src/__tests__/self-healing.test.ts

@@ -2360,7 +2360,7 @@ describe("SelfHealingManager", () => {
       managerWithRecovery.stop();
     });
 
-    it("finalizes no-op in-review tasks with zero commits ahead", async () => {
+    it("finalizes no-op in-review tasks with zero commits ahead (including review-level-0 coordination tasks)", async () => {
       const enqueueMerge = vi.fn();
       const managerWithRecovery = new SelfHealingManager(store, {
         rootDir: "/tmp/test-project",
@@ -2386,6 +2386,7 @@ describe("SelfHealingManager", () => {
             paused: false,
             status: null,
             worktree: "/tmp/test-project/.worktrees/fn-500",
+            reviewLevel: 0,
             steps: [{ name: "Ship it", status: "done" }],
             workflowStepResults: [{ id: "ws-1", status: "passed", phase: "pre-merge" }],
             mergeDetails: undefined,
@@ -2400,6 +2401,7 @@ describe("SelfHealingManager", () => {
             status: null,
             mergeRetries: 0,
             worktree: "/tmp/test-project/.worktrees/fn-500",
+            reviewLevel: 0,
             steps: [{ name: "Ship it", status: "done" }],
             workflowStepResults: [{ id: "ws-1", status: "passed", phase: "pre-merge" }],
             mergeDetails: { mergeConfirmed: true, noOpMerge: true },