fix(live): quantizzazione amount senza artefatti float (Decimal) — 72*0.001=0.07200000000000001 causava 'Invalid params' su Deribit

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/live/execution.py

@@ -17,6 +17,7 @@ from __future__ import annotations
 
 import time
 from dataclasses import dataclass, field
+from decimal import Decimal
 from typing import Any
 
 from src.live.cerbero_client import CerberoClient
@@ -46,6 +47,13 @@ def settlement_currency(instrument: str) -> str:
     return instrument.split("-")[0].split("_")[0]
 
 
+def _quantize_step(value: float, step: float, mn: float) -> float:
+    """Arrotonda `value` al multiplo di `step` SENZA artefatti float
+    (es. 72*0.001 = 0.07200000000000001 → Deribit 'Invalid params')."""
+    n_steps = round(value / step)
+    return float(max(n_steps * Decimal(str(step)), Decimal(str(mn))))
+
+
 def notional_to_amount(instrument: str, notional_usd: float,
                        price: float | None = None) -> float:
     """Notional USD → `amount` Deribit, arrotondato allo step e clampato al minimo.
@@ -59,10 +67,10 @@ def notional_to_amount(instrument: str, notional_usd: float,
         units = notional_usd / price          # base-coin richiesti
         if units < step / 2:
             return 0.0
-        return max(round(units / step) * step, mn)
+        return _quantize_step(units, step, mn)
     if notional_usd < step / 2:
         return 0.0
-    return max(round(notional_usd / step) * step, mn)
+    return _quantize_step(notional_usd, step, mn)
 
 
 @dataclass