Convert an amount with a caller-supplied FX rate (no live feed). Call when you need this bounded result before handing data to a downstream tool. Returns deterministic machine-readable JSON for $0.001 USDC via x402 on Base. Honest limit: execution uses only caller-supplied data in memory; it does not call a paid upstream, retain input, search the web, read a live RPC, or invoke a media generator. Results are advisory; the caller owns authorization, budgets, and production controls.