fix(sender): remove busy-wait between reply arrival and next probe

The session sender loop only blocked in the select immediately after
sending a probe. When the reflected reply arrived, that select returned
after one RTT, the reply was drained at the top of the loop, and the
loop then spun on the zero-timeout select until the next 1-second
deadline: a pure busy-wait costing (interval - RTT) of one core per
interval, i.e. roughly one full core per sender session at the default
probe rate.

Move the wait out of the send branch: after draining replies and the
endtime check, block on select until the next probe deadline, or until
endtime once all probes have been sent. A reply still wakes the select
early and is drained normally, so timing, sequencing, statistics and
log output are unchanged.

Loopback validation (10 probes, -i 1000, local responder):
  original: wall 9.07s, CPU 9.06s (100% of a core)
  patched:  wall 9.09s, CPU 0.07s (~1%)
0.0% loss, probes exactly 1s apart, reply log lines unchanged.
Full test suite: 12 passed.

Author: phr0stb1t3

src/twampy/__main__.py

@@ -349,13 +349,22 @@ def run(self):
                 log.info("Sent to %s [sseq=%d]", self.remote_addr, idx)
 
                 idx = idx + 1
-                if schedule > t1:
-                    r, w, e = select.select([self.socket], [], [], schedule - t1)
 
             if t1 > endtime:
                 log.info("Receive timeout for last packet (don't wait anymore)")
                 self.running = False
 
+            # Block until the next probe deadline (or wake early when a reply
+            # arrives). Waiting only right after a send is not enough: the
+            # arriving reply makes that select return after one RTT, and the
+            # loop then spins on the zero-timeout select above until the next
+            # deadline, burning (interval - RTT) of CPU every interval.
+            if self.running:
+                wait_until = schedule if idx < self.count else endtime
+                timeout = wait_until - now()
+                if timeout > 0:
+                    select.select([self.socket], [], [], timeout)
+
         self.stats.dump(idx)