Remove EUR/USD support — XAU/USD-only codebase

EUR/USD never had a validated edge (2026-07-11/12 verdict: no config
passed validation on M5/M15/H1 at 1-pip cost; cost-to-volatility root
cause). Removed: EurUsdStrategyConfig, pair_signals/eurusd.py, EUR
gates 6h + weekend/day filters + strategy marker + trade geometry in
signal_aggregator, EUR pair registry/aliases in main.py and
gpu_backtest.py, EUR/USD news pair mappings, test_all.py Module 10.
Generic multi-currency news keyword infra kept (GBP/JPY/EUR keyword
table is not pair code). Full EUR history remains at 7d3fd95.

Suite: 162/162 green (was 187; 25 EUR-specific tests removed).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015kby7M9JwpYi8NQT2yoxAi

news_engine.py

@@ -673,8 +673,7 @@ class NewsEngine:
     @staticmethod
     def _get_affected_pairs(currency: str) -> List[str]:
         _MAP = {
-            "USD": ["XAU/USD", "EUR/USD"],
+            "USD": ["XAU/USD"],
             "XAU": ["XAU/USD"],
-            "EUR": ["EUR/USD"],
         }
         return _MAP.get(currency, [])