Every mutation is a pure function over a draft dataset, so the write
surface is unit-tested without React. Money amounts (income, purchase
totals, split values, schedule slices, fixed/percent rule amounts) are
rounded to their currency's minor unit before they land in the draft,
per CLAUDE.md's round-before-persisting rule -- actions.ts is the
single boundary every UI mutation passes through. FxRate.baseUnitsPerOne
is deliberately exempt: it is stored at six decimal places, not money.
Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
