chore: add changeset for dataset-measure currency

Author: os-zhuang

.changeset/dataset-measure-currency.md

@@ -0,0 +1,12 @@
+---
+"@objectstack/spec": minor
+"@objectstack/service-analytics": minor
+---
+
+Propagate a dataset measure's declared currency to the analytics result field.
+
+Adds an optional `DatasetMeasure.currency` (ISO 4217) on the semantic layer and
+carries it onto each measure result field alongside `label`/`format`, so a
+currency-aware client (Intl symbol) can render `¥1,234` / `$616,000` from a real
+currency code instead of a plain number or a `$` baked into `format`. Additive
+and optional — existing datasets are unaffected.