adding USDT with 18 decimals gets 1:1 anchorPerMinted despite 6 decimals for IST

[dckc]

In ollinet, I added a PSM for USDT with decimalPlaces: 18 using startPSM() as usual, not realizing that it sets anchorPerMinted 1:1 irrespective of decimalPlaces:

agoric-sdk/packages/inter-protocol/src/proposals/startPSM.js

Line 86 in 1b3cbd2

anchorPerMinted: makeRatio(100n, anchorBrand, 100n, stable),

I tried to trade 0.2 USDT for IST and the trade failed with Request would exceed mint limit, which puzzled me, because I set the mint limit to 123456_000_000n. The mint limit is in uist, which prompted me to realize that this trade was asking for a lot of IST (roughly 2e11 = 0.2 * 1e18/1e6).

cc @warner @rowgraus @turadg