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).

Claude-Session: https://claude.ai/code/session_015kby7M9JwpYi8NQT2yoxAi

backtest/research/calendar_store.py

@@ -6,7 +6,7 @@ alignment (see scraper docstring).
 
 Key API:
     cal = load_calendar()                          # full DataFrame, `utc` column
-    ev  = precise_events(cal, ["USD", "EUR"])      # high-impact, minute-accurate
+    ev  = precise_events(cal, ["USD"])             # high-impact, minute-accurate
     mask = event_window_mask(bar_index, ev.utc, before, after)
 """
 from pathlib import Path
@@ -47,7 +47,7 @@ def load_calendar(path: Path = PARQUET) -> pd.DataFrame:
 
 def precise_events(
     cal: pd.DataFrame,
-    currencies: Iterable[str] = ("USD", "EUR"),
+    currencies: Iterable[str] = ("USD",),
     impacts: Iterable[str] = ("high",),
     require_values: bool = False,
 ) -> pd.DataFrame: