Add EUR/USD support with modular per-pair signal producer pattern

- Register EUR/USD in config (pip 0.0001, EurUsdStrategyConfig)
- Create pair_signals/ module: registry + dispatcher with per-pair
  config factories (xauusd.py, eurusd.py) for easy add/remove
- EUR/USD gets forex-tuned regime weights, London+NY session hours,
  XAU strategy gates disabled
- Add defense-in-depth display_name guards in signal_aggregator
- Rename telegram_bot.py → testing_telegram_bot.py with separate
  bot token, both pairs with dedicated button rows
- Add EUR/USD to news engine affected pairs map
- 9 new tests (146 total, all passing)
- Zero changes to engine.py, data_provider.py, main.py, backtest/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

news_engine.py

@@ -541,7 +541,8 @@ class NewsEngine:
     @staticmethod
     def _get_affected_pairs(currency: str) -> List[str]:
         _MAP = {
-            "USD": ["XAU/USD"],
+            "USD": ["XAU/USD", "EUR/USD"],
             "XAU": ["XAU/USD"],
+            "EUR": ["EUR/USD"],
         }
         return _MAP.get(currency, [])