Five independent tests at 1-pip cost (M15 wide-open both windows,
transitioning-slice OOS, mean-reversion trade sims, forward-return
screens on M15+M5, H1 both windows) — nothing clears cost in both
2024-25 and 2025-26. Root cause: spread is ~10% of M15 ATR on EUR.
EUR stays out of scan_pairs; open avenues documented in docstring.
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.