Water chemistry: suggest salt additions automatically from source + target profile #10

Source

User feedback (forum, v0.3, 2026-05-19) — Arwen. Reference behaviour: moneaudebrassage.fr.

Problem

Today water chemistry runs forward only: the brewer enters grams of gypsum / calcium chloride / etc. and the app shows the resulting ion profile (`computeWaterAdditions` in packages/calc/src/water-additions.ts).

What the brewer actually wants on brew day is the inverse: "my source water is this, my target profile is Burton — tell me how much of each salt to add." The current flow requires guess-and-check, or leaving Werb for another tool.

Requested behaviour

• Given source + target ion profiles, compute a recommended set of additions: gypsum, calcium chloride, epsom, table salt, baking soda.
• Show the residual delta per ion after applying the suggested additions, so the brewer sees how close the match actually is.
• The brewer can accept the suggestion as a starting point and then tweak individual salt values manually (preserves the current forward flow).

Notes

• Likely structure: a separate solver in `packages/calc/` (constrained least-squares over the salt → ion contribution matrix), contract-first with its own input/output JSON Schema per the SPEC convention.
• UI lives in apps/desktop/src/screens/Recipe/WaterChemistrySection.tsx, as a "Suggest additions" button next to the target picker.

Related

• Water chemistry: load source profiles (BeerJSON water + moneaudebrassage.fr API) #9 (source-profile loading from BeerJSON / moneaudebrassage API) — pairs naturally; once a source can be loaded in one tap, salt suggestion is the one-shot completion.
• Burton target = Burton source doesn't show as a match (separate issue, see below).