OptionalmaxSDK-side fee cap, 1e18-scaled fraction. Advisory, never an on-chain guarantee: no
MUSD write path takes a fee cap parameter, so the SDK reads, compares, then sends, and
the governable rate can move in between (MK-011). See docs/03-core-api.md §4.
Parameters for MusdClient.borrow: the MUSD to draw (+ optional fee cap).