Add pair_signals per-pair producer registry

Ported from feature/eurusd-support: PAIR_PRODUCERS registry with
make_config() factories per pair. xauusd.make_config is a pure
PlatformConfig-defaults wrapper (equality with the inline construction
is enforced by test); eurusd.make_config carries the donor branch's
regime weights and gates as a pre-realism baseline.

Adds 4 registry tests. 154/154 pass.

pair_signals/__init__.py

@@ -0,0 +1,52 @@
+"""
+Per-pair signal producers.
+
+Each pair module exposes:
+    make_config(primary_tf, confirmation_tfs) -> PlatformConfig
+
+To add a new pair: create pair_signals/<pair>.py with make_config(),
+then register it in this file.
+"""
+
+from typing import Optional, List, Dict, Callable
+
+from config import PlatformConfig, Timeframe
+from signal_aggregator import SignalAggregator, SignalReport
+from data_provider import ForexDataProvider
+
+PAIR_PRODUCERS: Dict[str, Callable] = {}
+
+
+def register_pair(symbol: str, config_factory: Callable):
+    PAIR_PRODUCERS[symbol] = config_factory
+
+
+def get_supported_pairs() -> List[str]:
+    return list(PAIR_PRODUCERS.keys())
+
+
+def produce_signal(
+    symbol: str,
+    primary_tf: Timeframe,
+    data_provider: ForexDataProvider,
+    confirmation_tfs: Optional[List[Timeframe]] = None,
+) -> SignalReport:
+    config_factory = PAIR_PRODUCERS.get(symbol)
+    if config_factory is None:
+        raise ValueError(
+            f"No signal producer registered for {symbol}. "
+            f"Available: {list(PAIR_PRODUCERS.keys())}"
+        )
+
+    config = config_factory(primary_tf, confirmation_tfs)
+    aggregator = SignalAggregator(config)
+    aggregator.data_provider = data_provider
+    return aggregator.analyze_pair(symbol=symbol)
+
+
+from pair_signals import xauusd as _xau_mod
+from pair_signals import eurusd as _eur_mod
+from config import XAUUSD, EURUSD
+
+register_pair(XAUUSD, _xau_mod.make_config)
+register_pair(EURUSD, _eur_mod.make_config)