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

test_ws_dual.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-"""Test whether Twelve Data allows 2 simultaneous WebSocket connections for EUR/USD.
+"""Test whether Twelve Data allows 2 simultaneous WebSocket connections for XAU/USD.
 
 Uses the twelvedata SDK which handles the WS protocol correctly.
 Runs connection 1, then adds connection 2 after 10s, monitors for 30s total.
@@ -11,7 +11,7 @@ import time
 from twelvedata import TDClient
 
 API_KEY = "f563b22ffdfd4924955decbe1a8c8701"
-SYMBOL = "EUR/USD"
+SYMBOL = "XAU/USD"
 DURATION = 30
 
 results = {