SelectControl: 32px trigger scale + option-group divider in the theme picker

docs/ux-second-pass.md

@@ -15,15 +15,17 @@ record the resolution into the contract (`docs/architecture/09`+`10` and the rel
   pane widths the long trigger labels ("FiveThirtyEight", "Urban Institute") may crowd it.
   Council questions: does the picker deserve header prominence (the "transform your chart"
   showcase) or settings-cluster placement (a persistent global pref); should the popover
-  registry learn nesting; 16 flat options — group presets under a heading?
+  registry learn nesting; should the preset block also carry a group _heading_ (a
+  divider now bounds it — 2026-06-12, user-decided; `SelectControl` `dividerBefore`)?
 
 - **"Edit themes…" action row inside the value picker** (`LivePreview.tsx` —
   ChartThemeControl). A non-value action lives inside a single-select disclosure (the
   VS Code theme-picker pattern), placed after the custom-themes block and before the
   preset roster (first-use feedback: at the very bottom it was invisible without
-  scrolling). Council questions: should an action be visually separated from the
-  values (divider, distinct styling); is a mid-list row that opens a modal instead of
-  selecting surprising to AT users?
+  scrolling). The ours-vs-presets divider (2026-06-12) now bounds it below; remaining
+  council questions: should the action row itself be styled distinctly from the value
+  rows; is a mid-list row that opens a modal instead of selecting surprising to AT
+  users?
 
 - **Theme Builder config editor is a plain textarea** (`ThemeBuilderModal.tsx`). Monaco
   (with the Vega-Lite config schema for completions) would match the main editor but is