Task: Keep each FX convention's ORE id

An FX convention keeps the id its document gave it, so every reference to it by that id resolves and the conventions export writes it back unchanged.

• Every FX convention in the corpus is stored under its own id, including the 600 or more that do not end in -FX-CONVENTIONS.
• A currency pair can hold more than one FX convention, as FX-BOE-USD-CAD and USD-CAD-FX do in one file.
• ores_refdata_validate_convention_id_fn resolves an FX convention by its stored id, and the conventions round trip asserts the id instead of documenting its loss.
• The document store writes FX conventions as world data, added only when the tenant lacks them, so a run import no longer skips them; the run store test expects none skipped.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

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