Function is_well_formed_fx_key 

fn is_well_formed_fx_key(key: &str) -> bool

Whether key is a well-formed FX fixing key: "FX-{BASE}{QUOTE}" with both halves real currency codes, the spelling FxIndex::canonical_name produces.

The FX namespace rule, stated once. FX fixings are a distinct namespace with no registry behind it — the index registry holds rate and price indices only — so an FX key is checked for its shape rather than for membership. This is a recorded exemption, not an unchecked path: a key that is neither a registered rate index nor of this shape is refused. Retirable the day an FX index registry exists, at which point membership replaces shape here.