fix(rustproxy-metrics): retain inactive per-IP metric buckets briefly to capture final throughput before pruning

test/test.throughput.ts

@@ -188,10 +188,12 @@ tap.test('TCP forward - real-time byte tracking', async (tools) => {
   const byRoute = m.throughput.byRoute();
   console.log('TCP forward — throughput byRoute:', Array.from(byRoute.entries()));
 
-  // After close, per-IP data should be evicted (memory leak fix)
+  // After close, per-IP buckets are retained briefly for final throughput sampling,
+  // but active connection counts must already be zero.
   const byIPAfter = m.connections.byIP();
   console.log('TCP forward — connections byIP after close:', Array.from(byIPAfter.entries()));
-  expect(byIPAfter.size).toEqual(0);
+  expect(byIPAfter.size).toBeGreaterThan(0);
+  expect(Array.from(byIPAfter.values()).every((count) => count === 0)).toEqual(true);
 
   await proxy.stop();
   await tools.delayFor(200);