Chart builder: drop unparseable expressions mid-edit, polite glyphed inline feedback

docs/architecture/10-interaction-and-feedback.md

@@ -368,6 +368,19 @@ bullet is the contract; cite it, not the source.)_
 other shows the text visually with no live role. Two live regions would announce the same
 message twice.
 
+**Resolved — inline _live_ validation feedback is polite, glyphed, and field-linked.** A
+validator that re-checks on **every keystroke** (the Chart Builder expression inputs — a
+calculated field, a filter in expression mode) is **not** an assertive `alert`: that would
+interrupt on each character (APG _Alert_ "avoid frequent interruptions"; WCAG 2.2.4). It is a
+**polite `role="status"`** line, and severity reads from a **status glyph** (round error /
+triangle warning, arch 09 §5.2) **plus** colour — never colour alone (§3; WCAG 1.4.1), exactly
+like the guidance warnings above it. The owning `<input>` carries `aria-invalid` for the state
+and `aria-describedby` pointing at the message node so the text is available on focus, not only
+when it changes (GOV.UK _error-message_ field association). Contrast the editor's **render**
+error, which is a discrete, post-debounce result and stays the single `alert` of the rule
+above. _(Consulted via /council → WAI-ARIA APG Alert, GOV.UK error-message, NN/g #9. This
+bullet is the contract; cite it, not the source.)_
+
 **Resolved — feature-modal dismissal & initial focus.** A feature modal (Datasets, Chart
 Builder) is a **passive** `dialog-modal`: dismissed by the close
 button, Escape, or a backdrop click (a passive modal carries no in-flight transaction, so