feat(gateway): add configurable lane queue mode, cap, and overflow

src/gateway/lane-queue.test.ts

@@ -191,4 +191,61 @@ describe('LaneQueue', () => {
     const r2 = await queue.enqueue('lane-a', async () => 'second');
     expect(r2).toBe('second');
   });
+
+  it('steer mode keeps only the most recent pending request', async () => {
+    const queue = new LaneQueue({ mode: 'steer' });
+    let resolveFirst!: () => void;
+    const firstBlocks = new Promise<void>((r) => { resolveFirst = r; });
+
+    const p1 = queue.enqueue('lane-a', async () => {
+      await firstBlocks;
+      return 'active';
+    });
+    const p2 = queue.enqueue('lane-a', async () => 'old-pending');
+    const p3 = queue.enqueue('lane-a', async () => 'latest-pending');
+
+    await expect(p2).rejects.toThrow('Superseded by newer request');
+    resolveFirst();
+
+    await expect(p1).resolves.toBe('active');
+    await expect(p3).resolves.toBe('latest-pending');
+  });
+
+  it('drop_new overflow rejects newest request when cap is reached', async () => {
+    const queue = new LaneQueue({ cap: 1, overflow: 'drop_new' });
+    let resolveFirst!: () => void;
+    const firstBlocks = new Promise<void>((r) => { resolveFirst = r; });
+
+    const p1 = queue.enqueue('lane-a', async () => {
+      await firstBlocks;
+      return 'active';
+    });
+    const p2 = queue.enqueue('lane-a', async () => 'pending-1');
+    const p3 = queue.enqueue('lane-a', async () => 'pending-2');
+
+    await expect(p3).rejects.toThrow('Lane queue full (drop_new)');
+    resolveFirst();
+
+    await expect(p1).resolves.toBe('active');
+    await expect(p2).resolves.toBe('pending-1');
+  });
+
+  it('drop_old overflow evicts oldest pending request when cap is reached', async () => {
+    const queue = new LaneQueue({ cap: 1, overflow: 'drop_old' });
+    let resolveFirst!: () => void;
+    const firstBlocks = new Promise<void>((r) => { resolveFirst = r; });
+
+    const p1 = queue.enqueue('lane-a', async () => {
+      await firstBlocks;
+      return 'active';
+    });
+    const p2 = queue.enqueue('lane-a', async () => 'old-pending');
+    const p3 = queue.enqueue('lane-a', async () => 'new-pending');
+
+    await expect(p2).rejects.toThrow('Lane queue overflow (drop_old)');
+    resolveFirst();
+
+    await expect(p1).resolves.toBe('active');
+    await expect(p3).resolves.toBe('new-pending');
+  });
 });