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/decision_engine.py

@@ -31,6 +31,17 @@ from .cadence import Action, CadenceControlPlane
 from .domain import StrictModel, Symbol, typed
 
 
+# Stablecoins / pegged assets that must NEVER be selected as a trade asset.
+# MUST equal nautilus_event_trader.py:_STABLECOIN_SYMBOLS (BLUE removes these from
+# prices_dict before selection, :3906) — asserted by test_violet_decision_engine.
+# Following BLUE in all regards: picking stays muted-IRP as BLUE, this is BLUE's
+# separate exclusion gate around it.
+STABLECOIN_SYMBOLS = frozenset({
+    "USDCUSDT", "BUSDUSDT", "FDUSDUSDT", "USDTUSDT", "TUSDUSDT",
+    "DAIUSDT", "FRAXUSDT", "USDDUSDT", "USTCUSDT", "EURUSDT",
+})
+
+
 class ShadowDecision(StrictModel):
     """One muted decision — what BLUE *would* do this scan. Never executed."""
 
@@ -107,6 +118,8 @@ class VioletDecisionEngine:
             if px <= 0:
                 continue
             sym = str(asset).upper()
+            if sym in STABLECOIN_SYMBOLS:        # BLUE :3906 — never a trade asset
+                continue
             hist = self._history.get(sym)
             if hist is None:
                 hist = deque(maxlen=maxlen)