fix(a11y): make every text colour clear WCAG AA on every ramp

a11y.md flagged --yj-text-tertiary on --yj-bg-surface as 'borderline
(~4.1:1) but that needs a real measurement', and plan 007 parked it as
'worth measuring before planning'. Measured, against the rendered app
and then across all three background ramps: it failed AA in nine of
twelve text/surface combinations, as low as 2.31:1 on dark's overlay
and 2.55:1 on light's -- the app's most-used secondary text colour,
failing on every view. Not borderline. 110 failing nodes across twelve
views, now 0 of 659.

Three separate mechanisms, and only the first is the finding:

- The ramps. Tertiary is raised per ramp (#a6a6a6 dark, #949494 darker,
  #5c636a light), sized to the lightest surface it actually sits on and
  keeping its hue. Sizing it to bgOverlay too would need a grey lighter
  than secondary, so bgOverlay is documented as not a text surface and
  the one component that put text there uses primary.
- The avatar generator. hsl(hue, 45%, 35%) behind white initials failed
  for 35 of the 360 hues -- the yellow-green band -- so which artists
  were unreadable depended on how their names hashed. The two a sweep
  found were not the finding. 32% clears every hue.
- Jobs' local #ff6b6b, at 4.15:1 on elevated.

Pinned by a unit test over the palette table rather than a DOM sweep:
the ramps are pure data, and checking only what happens to be on screen
is exactly how the light ramp went unexamined. Note that make ui-visual
cannot see any of this -- the component tier renders the fallbacks,
because theme-store sets :root only in the real app.

frontend/src/store/theme-store.ts

@@ -20,7 +20,7 @@ type Subscriber = () => void;
  * Shade palettes keyed by BackgroundShade.
  * Each defines the base grayscale ramp used throughout the UI.
  */
-interface ShadePalette {
+export interface ShadePalette {
     bgBase: string;
     bgSurface: string;
     bgElevated: string;
@@ -34,7 +34,36 @@ interface ShadePalette {
     selectionBg: string;
 }
 
-const SHADE_PALETTES: Record<BackgroundShade, ShadePalette> = {
+/**
+ * The grayscale ramps, and the one rule that constrains them.
+ *
+ * `textTertiary` used to be `#888888` on both dark ramps and `#868e96`
+ * on the light one, which `a11y.md` flagged as "borderline" from a hand
+ * calculation and parked as "worth measuring before planning".
+ * Measured, against the rendered app and then across all three ramps:
+ * it failed WCAG AA in **nine of twelve** text/surface combinations,
+ * as low as 2.31:1 on dark's overlay and 2.55:1 on light's. Not
+ * borderline — the app's most-used secondary text colour, failing on
+ * every view, and the light ramp (which the audit never considered) was
+ * the worst of the three.
+ *
+ * So: **every text colour clears 4.5:1 against every surface it can sit
+ * on**, and `theme-contrast.test.ts` computes that from this table
+ * rather than trusting it. Two things decided the values.
+ *
+ * `bgOverlay` is not a text surface on the dark ramp. Sizing tertiary
+ * to clear 4.5 against `#495057` needs `#c0c0c0`, which is *lighter
+ * than secondary* — an inverted hierarchy is a worse answer than the
+ * problem. Tertiary is sized to `bgElevated` there, and the one place
+ * that did put text on the overlay (the downloads notice) uses
+ * `textPrimary`, which clears 8.18:1.
+ *
+ * And the hue is kept. The light ramp's tertiary is a blue-grey, so it
+ * darkens along its own hue to `#5c636a` rather than flattening to a
+ * neutral that would have passed just as well and looked like a
+ * different palette.
+ */
+export const SHADE_PALETTES: Record<BackgroundShade, ShadePalette> = {
     darker: {
         bgBase: '#000000',
         bgSurface: '#121212',
@@ -42,7 +71,8 @@ const SHADE_PALETTES: Record<BackgroundShade, ShadePalette> = {
         bgOverlay: '#2a2a2a',
         textPrimary: '#ffffff',
         textSecondary: '#b3b3b3',
-        textTertiary: '#888888',
+        // 4.05:1 on bgOverlay at #888888.
+        textTertiary: '#949494',
         border: '#333333',
         borderSubtle: '#222222',
         hoverOverlay: 'rgba(255, 255, 255, 0.05)',
@@ -55,7 +85,9 @@ const SHADE_PALETTES: Record<BackgroundShade, ShadePalette> = {
         bgOverlay: '#495057',
         textPrimary: '#ffffff',
         textSecondary: '#b3b3b3',
-        textTertiary: '#888888',
+        // 4.35:1 on bgSurface and 3.25:1 on bgElevated at #888888 — the
+        // measured version of the audit's estimate, on every view.
+        textTertiary: '#a6a6a6',
         border: '#444444',
         borderSubtle: '#333333',
         hoverOverlay: 'rgba(255, 255, 255, 0.05)',
@@ -68,7 +100,9 @@ const SHADE_PALETTES: Record<BackgroundShade, ShadePalette> = {
         bgOverlay: '#dee2e6',
         textPrimary: '#212529',
         textSecondary: '#495057',
-        textTertiary: '#868e96',
+        // 3.32:1 at best and 2.55:1 at worst at #868e96 — the light ramp
+        // failed on all four of its own surfaces.
+        textTertiary: '#5c636a',
         border: '#ced4da',
         borderSubtle: '#dee2e6',
         hoverOverlay: 'rgba(0, 0, 0, 0.05)',