UX second pass: icon-only header, pane-toggle cue, schema columns, facet cleanup

docs/architecture/09-visual-design.md

@@ -210,8 +210,11 @@ UI by swapping one set of values. Borrowed from Carbon's layering model:
   action is filled. Worked examples: the **editor toolbar** (Publish is the lone
   primary; Extract/Revert are secondary, and collapse to icons when narrow — see
   [arch 10 §8](10-interaction-and-feedback.md)), and the **header** (Datasets /
-  Import / Export / About are ghost; a divider then sets off the soft-accent Donate
-  and the ghost theme toggle).
+  Import / Export / About are **icon-only** IconButtons per Carbon's UI-shell
+  header — global actions as a right-aligned icon row — with accessible names
+  that carry scope, e.g. "Export workspace" vs the preview's per-chart "Export";
+  a divider then sets off the soft-accent Donate, which keeps its text label —
+  a solicitation needs the word — and the ghost theme toggle).
 - **Hover is variant-specific:** filled buttons (primary/danger) **darken**
   (`--accent-hover` / a slight brightness drop); outlined/ghost buttons **gain a
   fill one elevation step above their surface** — on `--bg` → `--layer-01`, on a