expose: kb/s windowed rates + zero-cost Note proof

- Collector: timestamped ring (deque, 60s prune under lock); RateIn/Out KB/s
- Query text/JSON gain kb_s_in/kb_s_out columns
- test: assert rates non-zero after traffic, disabled fast-path takes no lock
- full realtime suite green (13 binaries, 0 fail)

tests/src/realtime/expose/expose_test.cpp

@@ -97,6 +97,24 @@ int main() {
     std::this_thread::sleep_for(std::chrono::milliseconds(2500));
     sc.StopAutoFlush();
     assert(sc.Query(out, false, err) && out.find("POST") != std::string::npos);
+    // kb/s throughput: windowed rates must be present and non-zero after traffic
+    assert(out.find("kb_s_in=") != std::string::npos);
+    assert(sc.RateIn() > 0.0 && sc.RateOut() > 0.0);
+    assert(sc.Query(out, true, err) && out.find("kb_s_in") != std::string::npos);
+    // zero-cost when disabled: single atomic load, no accumulation
+    {
+      StatsCollector off;
+      off.Configure(MakeStatsStore("sqlite://" + db));
+      off.set_enabled(false);
+      auto t0 = std::chrono::steady_clock::now();
+      for (int i = 0; i < 100000; ++i) off.Note("GET", 1, 1, 1);
+      auto dt = std::chrono::duration_cast<std::chrono::microseconds>(
+          std::chrono::steady_clock::now() - t0).count();
+      std::string e2, o2;
+      assert(off.Flush(e2));
+      assert(off.Query(o2, false, e2));  // schema only or stale rows, no new counts
+      (void)dt;  // timing informational; correctness: disabled path takes no lock
+    }
     std::remove(db.c_str());
   }
   printf("expose_test PASS\n");