sky: Midnight Sky's link is fitted to clear AA on the lightest Night sky

The dark theme's link was #609385 (luminance 0.2505), 2.28:1 on the lightest Night sky (0.081642). It is now
brightened in its own hue until its luminance is at least AA * (0.081642 + 0.05) - 0.05 = 0.5424: #8ad1bd (0.5468,
hue 163 degrees as before), 4.53:1 on the lightest Night sky, 5.43:1 on the darkest, 11.28:1 on the sign-in card
(was 5.68). Only the link changes in baked.ts; the user colours are unchanged. NIGHT_SKY_LIGHTEST is the Night backdrop
constant, and a test fails if the two drift. Dusk is untouched.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/app/features/scene/skyTheme.test.ts

@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest';
 import { atStandardTime } from './standardTime.testkit';
 import { baked } from '../../../sky/baked';
-import { AA, contrast, luminance } from '../../../sky/skyTokens.js';
+import { AA, NIGHT_SKY_LIGHTEST, contrast, luminance } from '../../../sky/skyTokens.js';
 import { Editor } from '../../components/editor/Editor.css';
 import { PageContent, PageNavContent } from '../../components/page/style.css';
 import { CallStatus } from '../call-status/styles.css';
@@ -59,13 +59,14 @@ describe('skyThemeId', () => {
 describe('ink of the theme a state picks, on bare sky', () => {
   // Per state: the picked theme, its text ink and link, and their WCAG ratios on the sky the scenes
   // draw (skyBackdrop, the worst case over its band). Text clears 4.5 in every state. Link clears
-  // on day and not on dusk or night: the links are derived in src/sky/skyTokens.js, outside the band, and these
-  // numbers are recorded as they are measured (Night's #609385 would clear 4.5 at a luminance of about 0.53 or more).
+  // on day and night and not on dusk: the links are derived in src/sky/skyTokens.js, outside the band, and these
+  // numbers are recorded as they are measured. Midnight's link is brightened in its own hue until it clears the
+  // lightest Night sky (NIGHT_SKY_LIGHTEST); Dusk's dark link would need a backdrop of about 0.455, and the dusk floor is lower.
   // 'auto' resolves from the clock to one of these three and has the same numbers.
   const cells: [FixedSkyState, string, string, string, string, string][] = [
     ['day', 'noonday-sky', '#342d25', '6.60', '#2a4f27', '4.55'],
     ['dusk', 'noonday-sky', '#342d25', '4.60', '#2a4f27', '3.17'],
-    ['night', 'midnight-sky', '#aeccff', '4.89', '#609385', '2.28'],
+    ['night', 'midnight-sky', '#aeccff', '4.89', '#8ad1bd', '4.53'],
   ];
 
   it.each(cells)(
@@ -83,11 +84,15 @@ describe('ink of the theme a state picks, on bare sky', () => {
     expect(ratio(ink, skyBackdrop(state))).toBeGreaterThanOrEqual(AA);
   });
 
-  it('link clears AA on day (the terrain camera sees only the lighter sky) and falls short on dusk and night', () => {
+  it('link clears AA on day and night and falls short on dusk', () => {
     const link = (s: FixedSkyState, id: keyof typeof SKY) => ratio(SKY[id].link, skyBackdrop(s));
     expect(link('day', 'noonday-sky')).toBeGreaterThanOrEqual(AA);
     expect(link('dusk', 'noonday-sky')).toBeLessThan(AA);
-    expect(link('night', 'midnight-sky')).toBeLessThan(AA);
+    expect(link('night', 'midnight-sky')).toBeGreaterThanOrEqual(AA);
+  });
+
+  it('keeps the lightest Night sky the link was fitted to equal to the Night backdrop (no drift)', () => {
+    expect(Math.abs(NIGHT_SKY_LIGHTEST - skyBackdrop('night').lightest)).toBeLessThan(1e-6);
   });
 
   it.each([
@@ -109,7 +114,7 @@ describe('ink of the theme a state picks, on bare sky', () => {
 describe('the plate, where text cannot rest on bare sky', () => {
   // Each theme's plate: fill, ink, link and the WCAG ratios of ink and link on the fill.
   const plates = [
-    ['midnight-sky', baked.midnight, '12.17', '5.68'],
+    ['midnight-sky', baked.midnight, '12.17', '11.28'],
     ['noonday-sky', baked.noonday, '6.70', '4.62'],
   ] as const;
 

src/app/features/scene/skyTheme.ts

@@ -5,10 +5,10 @@
 // Day and Dusk take the dark ink of Noonday Sky, Night the light ink of Midnight Sky. Each state's
 // drawn sky is shaped into a luminance band (skyBand.ts) in which that ink's TEXT clears 4.5:1 on
 // bare sky: Day and Dusk at least 0.2983 (dark ink), Night at most 0.0931 (light ink). skyTheme.test.ts
-// pins the measured text and link ratios. Link ink is not in the band: the Night link (#609385,
-// 2.36:1) and the dark links on Day and Dusk fall short, because the themes' link colours are
-// derived outside this code (src/sky/skyTokens.js); lightening the Night link to a luminance of
-// about 0.53 or more would clear it. The plate (skyPlate.css.ts) is still in the shell, pending
+// pins the measured text and link ratios. Link ink is not in the band: the Day and Dusk links are dark
+// and fall short of 4.5 where the sky is below about 0.455 (Dusk: 3.17), because the themes' link colours are
+// derived outside this code (src/sky/skyTokens.js). Midnight Sky's link is derived there against the lightest Night sky
+// (4.53 on it, 5.43 on the darkest). The plate (skyPlate.css.ts) is still in the shell, pending
 // its removal.
 import type { FixedSkyState, SkyState } from './skyState';
 

src/app/pages/auth/styles.css.ts

@@ -60,7 +60,7 @@ export const AuthFooter = style({
 // Noonday); the other themes (Light, Silver, Dark, Butter) take the fallback colours.
 // WCAG ratios of text / Register link on the card, per theme:
 //   Light, Silver, Dark, Butter  #b7c7d9 / #99c7ff on #0d1a26   10.21 / 10.03
-//   Midnight Sky                 #aeccff / #609385 on #0a090e   12.17 /  5.68
+//   Midnight Sky                 #aeccff / #8ad1bd on #0a090e   12.17 / 11.28
 //   Noonday Sky                  #342d25 / #2a4f27 on #b9b6af    6.70 /  4.62
 // The fallback link is set here because those themes' own links (Light 2.49:1, Silver 2.49:1) are
 // made for their own grounds, not this card; Dark and Butter already use the same hsl(213 100% 80%).

src/sky/baked.ts

@@ -115,7 +115,7 @@ export const baked = {
       '#677dff',
       '#6a926a',
     ],
-    link: '#609385',
+    link: '#8ad1bd',
   },
   noonday: {
     moment: {

src/sky/skyTokens.js

@@ -19,6 +19,10 @@ const luma = (c) => 0.2126 * c[0] + 0.7152 * c[1] + 0.0722 * c[2];
 export const GRADIENT_STOPS = 7;
 export const AA = 4.5; // WCAG 2.x AA, normal text
 
+// The lightest the Night sky draws behind the page (src/app/features/scene/skyState.ts, `skyBackdrop('night')`; a test in
+// skyTheme.test.ts fails if the two drift). Midnight Sky's link also sits on bare sky, so it must clear AA against this.
+export const NIGHT_SKY_LIGHTEST = 0.081642;
+
 // Where and when the theme is lit. Equinox, 45N on the prime meridian: noon sun at 45 degrees.
 export const MOMENTS = {
   noonday: { time: '2026-09-22T12:00:00Z', lat: 45, lon: 0 },
@@ -186,10 +190,10 @@ export function bakeSky(name) {
     bubbles.Own,
   ];
   const readable = (h) => grounds.every((g) => contrast(h, g) >= AA);
-  const fit = (albedo) => {
+  const fit = (albedo, ok = () => true) => {
     let key = 0.5;
     let h = hex(albedo, key);
-    for (let n = 0; n < 80 && !readable(h); n += 1) {
+    for (let n = 0; n < 80 && !(readable(h) && ok(h)); n += 1) {
       key *= dark ? 1.05 : 0.95;
       h = hex(albedo, key);
     }
@@ -207,7 +211,10 @@ export function bakeSky(name) {
     gradient,
     tokens,
     bubbles,
-    userColors: PIGMENTS.map(fit),
-    link: fit(dark ? ALBEDO.leafPale : ALBEDO.leaf),
+    userColors: PIGMENTS.map((p) => fit(p)),
+    // The dark link also has to clear AA on the bare Night sky: brightened, in its own hue, until it does.
+    link: dark
+      ? fit(ALBEDO.leafPale, (h) => luminance(h) >= AA * (NIGHT_SKY_LIGHTEST + 0.05) - 0.05)
+      : fit(ALBEDO.leaf),
   };
 }