test: de-flake volume-ratio test (real-clock dead zone, not plan 007)

compute_ratios bails when UTC now-16min precedes today's 08:00Z, i.e. nightly
~00:00-04:16 ET; the test's hardcoded 2026-07-06 bars made it KeyError at the
date rollover independent of any code change. Freeze the module clock to a
mid-session UTC time so the window is valid deterministically.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit/test_volume_ratio.py

@@ -32,7 +32,26 @@ class _Con:
         return self
 
 
-def test_compute_ratios_measures_and_skips_thin():
+def test_compute_ratios_measures_and_skips_thin(monkeypatch):
+    # Freeze the clock inside the stub's bar-date window. compute_ratios bails
+    # (`end <= start`) when UTC now-16min is before today's 08:00Z — true nightly
+    # from ~00:00-04:16 ET — so with hardcoded 2026-07-06 bars this test flaked at
+    # the date rollover regardless of the stub data. Pin a mid-session UTC clock.
+    import research.ingestion.volume_ratio as vr
+    from datetime import date as _date, datetime as _dt, timezone
+
+    class _FrozenDT:
+        @staticmethod
+        def now(tz=None):
+            return _dt(2026, 7, 6, 18, 0, 0, tzinfo=tz or timezone.utc)
+
+    class _FrozenDate:
+        @staticmethod
+        def today():
+            return _date(2026, 7, 6)
+
+    monkeypatch.setattr(vr, "datetime", _FrozenDT)
+    monkeypatch.setattr(vr, "date", _FrozenDate)
     r = compute_ratios(_Client(), ["GOOD", "DUST", "MISSING"])
     assert r["GOOD"] == 25.0                    # 12500 sip / 500 iex
     assert "DUST" not in r                      # sip sample below floor -> untrusted