VIOLET V3.1: BLUE stablecoin exclusion (parity fix)

Soak surfaced USDCUSDT being shorted. BLUE has a hardcoded exclusion gate around
its (muted-IRP) picking: _STABLECOIN_SYMBOLS removed from prices_dict pre-select
(nautilus_event_trader.py:24/3906). Replicate exactly: VioletDecisionEngine skips
the same 10 symbols in observe() so IRP never sees them. Following BLUE in all
regards (picking unchanged; this is BLUE's separate gate). Set-equality drift
guard vs BLUE source + never-selected test. 11 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

prod/clean_arch/violet/test_violet_decision_engine.py

@@ -8,8 +8,13 @@ sys.path.insert(0, "/mnt/dolphinng5_predict")
 
 import pytest
 
+import re
+from pathlib import Path
+
 from prod.clean_arch.violet.cadence import Action, CadenceControlPlane, INSTA_Q_NS, SCAN_Q_NS
-from prod.clean_arch.violet.decision_engine import ShadowDecision, VioletDecisionEngine
+from prod.clean_arch.violet.decision_engine import (
+    STABLECOIN_SYMBOLS, ShadowDecision, VioletDecisionEngine,
+)
 
 LOOKBACK = 5
 
@@ -101,6 +106,32 @@ def test_engine_holds_no_venue_or_kernel():
         assert not hasattr(e, attr)
 
 
+def test_stablecoin_set_matches_blue_exactly():
+    # Drift guard: VIOLET's exclusion set MUST equal BLUE's _STABLECOIN_SYMBOLS
+    # (nautilus_event_trader.py), parsed from source (no heavy import).
+    src = Path("/mnt/dolphinng5_predict/prod/nautilus_event_trader.py").read_text()
+    m = re.search(r"_STABLECOIN_SYMBOLS\s*=\s*frozenset\(\{(.*?)\}\)", src, re.DOTALL)
+    assert m is not None
+    blue = set(re.findall(r"'([A-Z0-9]+)'", m.group(1)))
+    assert STABLECOIN_SYMBOLS == blue
+
+
+def test_stablecoin_never_selected():
+    # Even with a strong short signal, a stablecoin must never be picked (BLUE :3906).
+    e = _engine()
+    assets = ["USDCUSDT", "BBBUSDT", "CCCUSDT"]
+    for s in range(8):
+        # USDC trends down hardest (would rank top by IRP if not excluded)
+        prices = [100.0 * (1 - 0.01 * s), 100.0 * (1 - 0.001 * s), 100.0 * (1 - 0.0005 * s)]
+        e.observe({"assets": assets, "asset_prices": prices}, scan_number=s + 1)
+    assert "USDCUSDT" not in e._history          # never accumulated
+    for k in range(3):
+        d = e.decide(now_ns=10**12 + k * SCAN_Q_NS, scan_number=20 + k,
+                     capital=69_000.0, vel_div=-0.20)
+        if d is not None:
+            assert d.asset != "USDCUSDT"
+
+
 def test_determinism_same_inputs_same_decision():
     e1, e2 = _engine(), _engine()
     _warm(e1); _warm(e2)