feat(gateway): monotonic clock helper (1879)

ingestor/connection/clock.py

@@ -0,0 +1,19 @@
+import time
+
+
+class MonotonicClock:
+    """Single monotonic reference for latency/timeout math (1879 §2).
+
+    Wall-clock (`now_wall`) is only for human-facing timestamps; all elapsed/
+    timeout math must use `elapsed()` so NTP re-sync can't produce negatives.
+    """
+
+    def __init__(self):
+        self._start = time.monotonic()
+
+    def elapsed(self) -> float:
+        return time.monotonic() - self._start
+
+    @staticmethod
+    def now_wall() -> float:
+        return time.time()

tests/unit/connection/test_clock.py

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+from ingestor.connection.clock import MonotonicClock
+
+
+def test_elapsed_is_non_negative_and_monotonic():
+    clk = MonotonicClock()
+    a = clk.elapsed()
+    b = clk.elapsed()
+    assert a >= 0 and b >= a  # never negative even if wall clock jumps back