palette: give the spectral mid and the envelope trace their own roles

Decouples the keyboard strip's mid stop from accent/secondary, which the
deep-teal darkening had inverted. Adds overlay/trace (#816AA6), the first
value to clear 3:1 against the waveform. Replaces the frozen-premise test.

src/core/ui/theme.h

@@ -1,14 +1,14 @@
 #pragma once
 // theme — the palette + type-scale core of the shared drawing kit: a ROLE-based color model
 // (bg/base, bg/panel, bg/cell, line/hairline, text/primary, text/dim, accent/primary,
-// accent/secondary, accent/tertiary, accent/hot, warn), an interaction-state model
-// (rest/hover/active/pressed/dragging/focus/disabled), and the WCAG contrast math that lets a
-// unit test prove every text-on-surface pair clears its floor.
+// accent/secondary, accent/tertiary, accent/hot, overlay/trace, warn), an interaction-state
+// model (rest/hover/active/pressed/dragging/focus/disabled), and the WCAG contrast math that
+// lets a unit test prove every text-on-surface pair clears its floor.
 //
 // Every role color is produced by roleColor() from ONE direction constants block (theme.cpp) —
 // the single point of change; no shell hardcodes a color, it asks by role. The spectral hue ramp
 // (spectralColor) lives here too so the keyboard strip derives its per-note hue from the same
-// source, anchored on the three accents (primary -> secondary -> tertiary).
+// source.
 
 #include <cstdint>
 
@@ -36,9 +36,11 @@ enum class Role {
     TextPrimary,   // labels, values
     TextDim,       // secondary / units
     AccentPrimary, // live / active / selected — where the punch lives (pastel lime)
-    AccentSecondary,// categorical role A (pastel teal) — a distinct KIND, never intensity
+    AccentSecondary,// categorical role A (deep teal) — a distinct KIND, never intensity
     AccentTertiary,// categorical role B (pastel purple) — a distinct KIND, never intensity
     AccentHot,     // hover / live / drag feedback (a brighter tint OF the primary accent)
+    OverlayTrace,  // a line/handle traced OVER an accent fill — reads against the fill AND the
+                   // surface behind it, which no accent role can do (they are all too light)
     Warn,          // clip / destructive (prune, delete) — reserved for byte-deleting states
 };
 
@@ -70,9 +72,9 @@ KitColor roleColor(Role role);
 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 accent/primary (low) -> accent/secondary (mid) -> accent/tertiary (high),
-// so the strip reads as an extension of the accent system rather than a separate flourish.
-// t is clamped to [0, 1].
+// 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].
 KitColor spectralColor(double t);
 
 // --- WCAG contrast (the "punch" rule, made testable) --------------------------