test(theme): widen distinguishability threshold headroom

The distinguishability threshold (10) sat only 2 units below the palette's
measured closest pair (coral/peach at 12.0), making it brittle to a minor
future nudge. Lower to 8: still fails the old palette's closest pair
(fog/storm at 7.55) while giving the current palette 4 units of headroom.
Also corrects the spec's lightness-change figure from -0.1 (computed from
rounded HSL) to the precise -0.18.

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

@@ -92,7 +92,7 @@ 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, holds lightness
-near-constant against today's values (measured average change: +4.1 saturation points, −0.1
+near-constant against today's values (measured average change: +4.1 saturation points, −0.18
 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

src/lib/utils/colors.test.ts

@@ -71,14 +71,15 @@ describe('dark note colours', () => {
 	});
 
 	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.
+		// The old palette failed this: fog and storm sat 7.55 apart, so the twelve
+		// colours read as one dark smudge. Threshold 8 clears that while leaving
+		// headroom above the current closest pair, coral/peach at 12.0.
 		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 ${nameB}`).toBeGreaterThan(10);
+				expect(distance, `${nameA} vs ${nameB}`).toBeGreaterThan(8);
 			}
 		}
 	});