docs(skills): teach the derived-measure fixed-currency refusal beside the two fixed-currency refusals

.changeset/28051-skills-derived-currency-refusal.md

@@ -0,0 +1,12 @@
+---
+'@objectstack/skills': patch
+---
+
+docs(skills): the skills teach the third `fixed`-currency refusal, a derived measure's own `currency` against its operands' one `fixed` code
+
+Clause-②: no
+Metadata-protocol: none
+
+- `objectstack-data` `rules/field-types.md` (Currency resolution) and `objectstack-ui` `rules/dashboards.md` (the dataset currency comment) taught two `fixed` refusals and missed the third: a derived `sum` or `difference` whose monetary operands all resolve to one `fixed` code refuses another `currency` at dataset compile, with `DATASET_INVALID` / 400. Both files now teach it beside the other two, with the shipped prescription: delete the measure's `currency` (the column takes the operands' code), or restate that code.
+- Paid for inside each file: `field-types.md` drops the `currency_code` naming convention its own example already shows; `dashboards.md` drops the retired `categoryGranularity` parenthetical, which the parse refusal and the Dataset-Bound Widgets paragraph already carry. Neither ratchet row moves.
+- No schema, key or behaviour changes.

content/docs/api/data-api.mdx

@@ -518,12 +518,18 @@ monetary measure column takes the measure's own `currency`; then, only when the
 field is `currencyMode: 'fixed'`, that field's `currencyConfig.defaultCurrency`; then the
 tenant default currency (the `localization.currency` setting), if one is set. A
 `dynamic` field (the default mode) never lends its `defaultCurrency` — only `fixed` does —
-so its column shows the tenant default. The cube query on this page carries no column
-`currency`.
+so its column shows the tenant default. A derived `sum` or `difference` whose monetary
+operands all resolve to one `fixed` code takes that code at the field's step; any other
+derived measure (a `ratio`, a `product`, operands in the tenant's currency) is monetary
+only by its own `currency`, else its column carries none. The cube query on this page
+carries no column `currency`.
 
 Over a `fixed` field the measure's `currency` may only restate the field's code. A
 different code is refused when the dataset compiles, with `DATASET_INVALID` / 400: delete
-the measure's `currency`, and the column takes the field's code. A derived measure whose
+the measure's `currency`, and the column takes the field's code. A derived `sum` or
+`difference` whose operands resolve to one `fixed` code is held to the same rule: a
+different `currency` is refused with `DATASET_INVALID` / 400 — delete the measure's
+`currency` (the column takes the operands' code), or restate it. A derived measure whose
 monetary operands resolve to two different `fixed` codes is refused the same way, whatever
 the operation: derive it from measures in one currency, or drop the operand in the other.
 Neither a `fixed` code against the tenant default nor a measure `currency` over a

content/docs/data-modeling/currency.mdx

@@ -178,8 +178,12 @@ that column. A `count` column never carries `currencyField`. See the
 - **A derived measure whose monetary operands resolve to two different `fixed` codes** is
   refused at compile with `DATASET_INVALID` / 400, whatever the operation. Derive it from
   measures in one currency, or drop the operand in the other. A derived `sum` or
-  `difference` over one `fixed` code carries that code, so a measure built on it is judged
-  the same way.
+  `difference` over one `fixed` code carries that code, on its result column too, so a
+  measure built on it is judged the same way.
+- **A derived `sum` or `difference` whose `currency` differs from its operands' one `fixed`
+  code** is refused at compile with `DATASET_INVALID` / 400: `currency: 'USD'` over two
+  measures fixed to `EUR` would print euros as dollars. Delete the measure's `currency`, and
+  the column takes the operands' code, or restate that code.
 - **Not judged:** a `fixed` code against the tenant currency, which compile does not know,
   and a measure `currency` over a `dynamic` field, which wins as the top of the currency
   chain.

skills/objectstack-data/rules/field-types.md

@@ -170,9 +170,10 @@ Stored as JSON on the parent row — no separate table / FK:
 `{ value, currency }`. Shown: `'fixed'`, `defaultCurrency`; `'dynamic'`
 (default), the tenant `localization.currency`, else a plain number (never
 `$`); `'record'`, each record's code in the `text` field `currencyField`
-names (by convention `currency_code`), with `valueDomain: 'iso_4217_currency'`. A
-dataset measure's own `currency` wins over a `'dynamic'` field; over a `'fixed'` one
-it may only restate the code, else `DATASET_INVALID` (objectstack-ui →
+names, with `valueDomain: 'iso_4217_currency'`. A
+dataset measure's own `currency` wins over a `'dynamic'` field; over a `'fixed'` one,
+or a derived sum/difference over one `'fixed'` code, it may only restate that code,
+else `DATASET_INVALID`: delete it, and the column takes the code (objectstack-ui →
 Dashboards). A `'fixed'` code is not judged against the tenant's. Switched a
 stored field to `'record'`? Backfill: `os migrate record-currency-codes`.
 

skills/objectstack-ui/rules/dashboards.md

@@ -282,8 +282,7 @@ The **dataset dimension carries the default; a widget's
 `options.dateGranularity` overrides it for that widget only.** Give a date
 dimension a `dateGranularity` and any presentation that selects it groups by that
 bucket server-side — without it every distinct timestamp becomes its own
-category, collapsing a 12-row seed into a 12-point flat line. (The old widget-level
-`categoryGranularity` was removed in the single-form cutover.)
+category, collapsing a 12-row seed into a 12-point flat line.
 
 <!-- os:check -->
 ```typescript
@@ -301,8 +300,9 @@ defineDataset({
 // `currencyMode: 'record'` amount refuses `currency`, and sum/avg/min/max
 // unless grouped by its code field; then the column carries
 // `currencyField` (the per-row code column); a `count` never does. A `fixed`
-// field refuses a different `currency` (delete it; restating is fine), and
-// a derived measure refuses operands in two `fixed` codes (derive from one
+// field, or a derived sum/difference over one `fixed` code, refuses another
+// `currency` (delete it, the column takes that code; restating is fine); a
+// derived measure refuses operands in two `fixed` codes (derive from one
 // currency or drop one). Not judged: `currency` over a `dynamic` field, a
 // `fixed` code against the tenant's.