Currency-aware decimal precision in /v1/convert and /v1/carat

Converted amounts now follow each currency's natural decimal places. Currencies with no minor unit (JPY, KRW, VND, and others) return whole numbers instead of trailing decimals; most currencies stay at 2 places, and a few (such as BHD and KWD) use 3. This affects the result field of /v1/convert and the per-gram prices in /v1/carat. The values are the same, just rounded to each currency's real precision. The exchange rate field is unchanged and keeps full precision. HTTP shapes and status codes are unchanged.