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/core/ui/theme.h

@@ -56,7 +56,14 @@ enum class InteractionState {
     Disabled,
 };
 
-// Text size classes for the WCAG floor: "Large" (>= ~18.66px, or >= ~14px bold) and UI-state
+// WCAG 2.1's large-scale thresholds, in px at 96dpi (18pt = 24px normal, 14pt = 18.66px bold).
+// A previous revision wrote the BOLD threshold as the normal one and both ~25% low, which let
+// 15px semibold self-classify as Large and clear a floor it was not entitled to — draw sites
+// that want the Large floor static_assert their font against these.
+inline constexpr double kLargeTextMinPx     = 24.0;
+inline constexpr double kLargeTextMinBoldPx = 18.66;
+
+// Text size classes for the WCAG floor: "Large" (>= 24px, or >= ~18.66px BOLD) and UI-state
 // indicators clear at 3:1; body text clears at 4.5:1 (WCAG 2.1 AA).
 enum class TextClass {
     Body,   // AA 4.5:1
@@ -73,14 +80,26 @@ KitColor roleColorState(Role role, InteractionState state);
 
 // Spectral hue ramp for the keyboard strip: maps normalized position t in [0, 1] (low note ->
 // high note) through a pastel lime -> teal -> purple arc, so the strip reads as part of the
-// palette rather than a separate flourish. The arc is its own three stops — a categorical
-// accent move must not be able to reorder it. t is clamped to [0, 1].
+// palette rather than a separate flourish. The MID stop is its own value; lo/hi remain aliases
+// of accent/primary and accent/tertiary, so a categorical accent move can still reorder the
+// ramp — testSpectralRampLuminanceIsMonotonic is the build-time catch, not the structure.
+// t is clamped to [0, 1].
 KitColor spectralColor(double t);
 
 // --- WCAG contrast (the "punch" rule, made testable) --------------------------
 
+// The effective opaque color of `over` drawn at `alpha` on top of `under` — the composition the
+// contrast math needs before it can judge a translucent fill. alpha is clamped to [0, 1]; the
+// result carries `under`'s alpha byte.
+KitColor compositeOver(const KitColor& over, const KitColor& under, double alpha);
+
+// The waveform's loop-span fill alpha. Named HERE rather than at its draw site so the contrast
+// test composes the same value the shell draws with (see compositeOver).
+inline constexpr double kLoopSpanFillAlpha = 0.20;
+
 // Relative luminance per WCAG 2.1 (sRGB linearization + 0.2126/0.7152/0.0722 weighting). Alpha
-// is ignored — a translucent overlay's effective color is the caller's to compose first.
+// is ignored — a translucent overlay's effective color is the caller's to compose first
+// (compositeOver).
 double relativeLuminance(const KitColor& c);
 
 // WCAG contrast ratio between two colors, in [1, 21]. Symmetric.