feat(FN-4973): complete Step 1 — add stale self-owned registry reconcile helper

Fusion-Task-Id: FN-4973
Fusion-Task-Lineage: 2edb5479-a01a-4775-b121-f4be32792e7a

packages/engine/src/__tests__/active-session-registry.test.ts

@@ -35,4 +35,31 @@ describe("activeSessionRegistry", () => {
 
     warnSpy.mockRestore();
   });
+
+  it("reconcileStaleSelfOwned returns no-entry when path is unregistered", () => {
+    expect(activeSessionRegistry.reconcileStaleSelfOwned("/tmp/missing", "FN-1")).toEqual({
+      reconciled: false,
+      reason: "no-entry",
+    });
+  });
+
+  it("reconcileStaleSelfOwned returns foreign-task for mismatched owner", () => {
+    activeSessionRegistry.registerPath("/tmp/w1", { taskId: "FN-2", kind: "executor", ownerKey: "FN-2" });
+
+    expect(activeSessionRegistry.reconcileStaleSelfOwned("/tmp/w1", "FN-1")).toEqual({
+      reconciled: false,
+      reason: "foreign-task",
+    });
+    expect(activeSessionRegistry.lookupByPath("/tmp/w1")?.taskId).toBe("FN-2");
+  });
+
+  it("reconcileStaleSelfOwned unregisters matching self-owned entry", () => {
+    activeSessionRegistry.registerPath("/tmp/w1", { taskId: "FN-1", kind: "executor", ownerKey: "FN-1" });
+
+    expect(activeSessionRegistry.reconcileStaleSelfOwned("/tmp/w1", "FN-1")).toEqual({
+      reconciled: true,
+      reason: "reconciled",
+    });
+    expect(activeSessionRegistry.lookupByPath("/tmp/w1")).toBeNull();
+  });
 });

packages/engine/src/active-session-registry.ts

@@ -10,6 +10,11 @@ export interface ActiveSessionRecord extends ActiveSessionRegistration {
   registeredAt: number;
 }
 
+export interface ReconcileStaleSelfOwnedResult {
+  reconciled: boolean;
+  reason: "no-entry" | "foreign-task" | "reconciled";
+}
+
 class ActiveSessionRegistry {
   private readonly records = new Map<string, ActiveSessionRecord>();
 
@@ -45,6 +50,19 @@ class ActiveSessionRegistry {
     return paths;
   }
 
+  reconcileStaleSelfOwned(worktreePath: string, expectedTaskId: string): ReconcileStaleSelfOwnedResult {
+    const record = this.lookupByPath(worktreePath);
+    if (!record) {
+      return { reconciled: false, reason: "no-entry" };
+    }
+    if (record.taskId !== expectedTaskId) {
+      return { reconciled: false, reason: "foreign-task" };
+    }
+
+    this.unregisterPath(worktreePath);
+    return { reconciled: true, reason: "reconciled" };
+  }
+
   clear(): void {
     this.records.clear();
   }