test(theme): replace lightness-band test with pairwise distinguishability

The lightness-band guard was looser than the palette it was meant to catch
(old band 0.0175 vs new 0.0246) and the muted-AA test never failed against
the old palette (5.08:1), so both docs' stated expectations and one spec
claim ("chroma roughly doubles") were measured falsehoods. Replace the band
test with an RGB-distance pairwise check, which does discriminate (old
closest pair fog/storm = 7.5, new closest pair coral/peach = 12), fix the
misleading test comment, and correct the spec/plan docs to match reality.

docs/superpowers/plans/2026-07-31-dark-palette-rework.md

@@ -82,7 +82,10 @@ describe('dark note colours', () => {
 - [ ] **Step 2: Run the tests to verify they fail**
 
 Run: `pnpm test:unit colors`
-Expected: FAIL. The muted-text test fails on several colours (the old `#4a2522` family sits too close to `#b3a695`), and the lightness-band test fails because the old values wander.
+Expected: FAIL. Only the two hex-pinning assertions fail (they still pin the old values). The AA
+tests are forward guards, not regressions of the old palette — the old palette actually clears
+both AA thresholds. The distinguishability test is the one that would fail on the old palette
+(its closest pair, `fog`/`storm`, sits at RGB distance 7.5).
 
 - [ ] **Step 3: Replace the dark palette**
 

docs/superpowers/specs/2026-07-31-dark-palette-design.md

@@ -91,10 +91,13 @@ html, body { accent-color: var(--accent-check); }
 
 ## 2. Note card colours (`src/lib/utils/colors.ts`)
 
-Replace `NOTE_COLORS_DARK`. Every entry keeps its light counterpart's hue, chroma roughly doubles
-against today's values, and lightness is held near-constant (~18%) so the twelve read as siblings —
-with blues and purples lifted ~3.5 points because they read darker than warm hues at equal HSL
-lightness.
+Replace `NOTE_COLORS_DARK`. Every entry keeps its light counterpart's hue, holds lightness
+near-constant against today's values (measured average change: +4.1 saturation points, −0.1
+lightness points), and nudges two crowded hues apart (`blossom` and `storm` shift toward pink
+and true blue respectively) so the twelve read as distinguishable siblings rather than a smudge.
+The visible improvement in dark mode comes from the base surfaces, the picker chips, the
+softened border, and the lifted gold — not from the card surfaces, which are close in chroma to
+the previous dark values.
 
 ```ts
 export const NOTE_COLORS_DARK: Record<NoteColor, { bg: string; label: string }> = {

src/lib/utils/colors.test.ts

@@ -24,10 +24,14 @@ describe('getNoteColor', () => {
 	});
 });
 
+function toRgb(hex: string): [number, number, number] {
+	const n = parseInt(hex.slice(1), 16);
+	return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
+}
+
 /** WCAG relative luminance. */
 function relativeLuminance(hex: string): number {
-	const n = parseInt(hex.slice(1), 16);
-	const channels = [(n >> 16) & 255, (n >> 8) & 255, n & 255].map((v) => {
+	const channels = toRgb(hex).map((v) => {
 		const s = v / 255;
 		return s <= 0.03928 ? s / 12.92 : Math.pow((s + 0.055) / 1.055, 2.4);
 	});
@@ -39,6 +43,12 @@ function contrastRatio(a: string, b: string): number {
 	return (Math.max(l1, l2) + 0.05) / (Math.min(l1, l2) + 0.05);
 }
 
+function rgbDistance(a: string, b: string): number {
+	const [ra, ga, ba] = toRgb(a);
+	const [rb, gb, bb] = toRgb(b);
+	return Math.hypot(ra - rb, ga - gb, ba - bb);
+}
+
 describe('dark note colours', () => {
 	// Must track [data-theme="dark"] in src/app.css.
 	const DARK_TEXT = '#ece3d3';
@@ -51,15 +61,25 @@ describe('dark note colours', () => {
 	});
 
 	it('keeps muted text above the AA threshold on every dark card', () => {
-		// This is the invariant the old palette broke: muted/struck-through text
-		// became unreadable on the lighter surfaces.
+		// The shipped dark theme failed this at 3.77:1 with its previous muted
+		// colour (#9a8e7e). This guard exists so a future palette edit cannot
+		// reintroduce it — it does not discriminate the old surfaces, which were
+		// darker and so scored higher against the lifted muted colour.
 		for (const [name, { bg }] of Object.entries(NOTE_COLORS_DARK)) {
 			expect(contrastRatio(bg, DARK_TEXT_MUTED), `${name} (${bg})`).toBeGreaterThanOrEqual(4.5);
 		}
 	});
 
-	it('holds the twelve surfaces within a narrow lightness band so they read as a family', () => {
-		const luminances = Object.values(NOTE_COLORS_DARK).map((c) => relativeLuminance(c.bg));
-		expect(Math.max(...luminances) - Math.min(...luminances)).toBeLessThan(0.03);
+	it('keeps every pair of dark surfaces distinguishable', () => {
+		// The old palette failed this: fog and storm sat 7.5 apart, so the twelve
+		// colours read as one dark smudge. Current closest pair is coral/peach at 12.
+		const entries = Object.entries(NOTE_COLORS_DARK);
+		for (let i = 0; i < entries.length; i++) {
+			for (let j = i + 1; j < entries.length; j++) {
+				const [[nameA, a], [nameB, b]] = [entries[i], entries[j]];
+				const distance = rgbDistance(a.bg, b.bg);
+				expect(distance, `${nameA} vs ${nameB}`).toBeGreaterThan(10);
+			}
+		}
 	});
 });