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 }> = {