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.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Harwinder Sekhon