palette: enlarge the region title into WCAG large class; repoint the grabbed envelope handle off hue

Corrects theme.h's large-text thresholds, names accent/secondary's real
binding limiter, and adds compositeOver so the loop-span fill's 2.25:1
under-floor pair is asserted rather than assumed.

src/shell/panel/panel_state.h

@@ -198,7 +198,9 @@ inline constexpr int kTooltipCharPx = 7;
 inline constexpr int kTooltipTextH  = 14;
 
 // Client area top to bottom: top toolbar | split body | bottom toolbar | footer.
-inline constexpr int kRegionHeaderHeight = 24;
+// 26, not 24: Font::RegionTitle's line box (19px em + Segoe UI's leading) is ~25px, and
+// DT_VCENTER clips to the rect.
+inline constexpr int kRegionHeaderHeight = 26;
 inline constexpr int kTabStripHeight     = 26;
 inline constexpr int kSplitDividerHeight = 3;
 inline constexpr int kFullHtBtnWidth     = 22;