Function resolve_fx_pair 

pub fn resolve_fx_pair(
    registry: &SymbolRegistry,
    from_currency: &str,
    to_currency: &str,
    available_symbols: &BTreeSet<String>,
) -> Result<Option<ConversionLeg>, ConversionError>

Resolve one directed FX pair from the available-symbol set.

A direct pair is preferred to an inverse pair. Within either orientation, canonical symbol order provides deterministic tie-breaking.