Eng-council sweep: export hygiene, dead code, helper dedup, declare vega-expression

src/core/expr-validate.ts

@@ -9,8 +9,8 @@
  * grammar. It also extracts the `datum.<field>` references so the UI can flag a typo
  * against the dataset's actual columns before the chart silently renders empty.
  *
- * `vega-expression` is a pure dependency already in the bundle (Vega pulls it in for
- * rendering), so importing it here adds nothing and keeps this module browser-free.
+ * `vega-expression` is declared as a direct dependency but adds no bundle weight —
+ * Vega already ships it for rendering; this import reuses the same copy.
  */
 
 import { parseExpression } from 'vega-expression';